diff --git a/crates/ruma-appservice-api/src/event/push_events/v1.rs b/crates/ruma-appservice-api/src/event/push_events/v1.rs
index 9bd8e97b..cedca421 100644
--- a/crates/ruma-appservice-api/src/event/push_events/v1.rs
+++ b/crates/ruma-appservice-api/src/event/push_events/v1.rs
@@ -2,6 +2,7 @@
 
 use ruma_api::ruma_api;
 use ruma_events::AnyRoomEvent;
+use ruma_identifiers::TransactionId;
 use ruma_serde::Raw;
 
 ruma_api! {
@@ -19,7 +20,7 @@ ruma_api! {
         ///
         /// Homeservers generate these IDs and they are used to ensure idempotency of results.
         #[ruma_api(path)]
-        pub txn_id: &'a str,
+        pub txn_id: &'a TransactionId,
 
         /// A list of events.
         pub events: &'a [Raw<AnyRoomEvent>],
@@ -31,14 +32,14 @@ ruma_api! {
 
 impl<'a> Request<'a> {
     /// Creates a new `Request` with the given transaction ID and list of events.
-    pub fn new(txn_id: &'a str, events: &'a [Raw<AnyRoomEvent>]) -> Self {
+    pub fn new(txn_id: &'a TransactionId, events: &'a [Raw<AnyRoomEvent>]) -> Self {
         Self { txn_id, events }
     }
 }
 
 impl IncomingRequest {
     /// Creates an `IncomingRequest` with the given transaction ID and list of events.
-    pub fn new(txn_id: String, events: Vec<Raw<AnyRoomEvent>>) -> IncomingRequest {
+    pub fn new(txn_id: Box<TransactionId>, events: Vec<Raw<AnyRoomEvent>>) -> IncomingRequest {
         IncomingRequest { txn_id, events }
     }
 
@@ -171,7 +172,7 @@ mod tests {
         let dummy_event = Raw::new(&dummy_event).unwrap();
         let events = vec![dummy_event];
 
-        let req = Request { events: &events, txn_id: "any_txn_id" }
+        let req = Request { events: &events, txn_id: "any_txn_id".into() }
             .try_into_http_request::<Vec<u8>>(
                 "https://homeserver.tld",
                 SendAccessToken::IfRequired("auth_tok"),
diff --git a/crates/ruma-client-api/src/r0/message/send_message_event.rs b/crates/ruma-client-api/src/r0/message/send_message_event.rs
index c90bdeb6..f5dfbbf5 100644
--- a/crates/ruma-client-api/src/r0/message/send_message_event.rs
+++ b/crates/ruma-client-api/src/r0/message/send_message_event.rs
@@ -2,7 +2,7 @@
 
 use ruma_api::ruma_api;
 use ruma_events::{AnyMessageEventContent, MessageEventContent};
-use ruma_identifiers::{EventId, RoomId};
+use ruma_identifiers::{EventId, RoomId, TransactionId};
 use ruma_serde::Raw;
 use serde_json::value::to_raw_value as to_raw_json_value;
 
@@ -31,7 +31,7 @@ ruma_api! {
         /// same access token; it will be used by the server to ensure
         /// idempotency of requests.
         #[ruma_api(path)]
-        pub txn_id: &'a str,
+        pub txn_id: &'a TransactionId,
 
         /// The event content to send.
         #[ruma_api(body)]
@@ -55,7 +55,7 @@ impl<'a> Request<'a> {
     /// [`Serialize`][serde::Serialize] implementation can fail.
     pub fn new<T: MessageEventContent>(
         room_id: &'a RoomId,
-        txn_id: &'a str,
+        txn_id: &'a TransactionId,
         content: &'a T,
     ) -> serde_json::Result<Self> {
         Ok(Self {
@@ -70,7 +70,7 @@ impl<'a> Request<'a> {
     /// content.
     pub fn new_raw(
         room_id: &'a RoomId,
-        txn_id: &'a str,
+        txn_id: &'a TransactionId,
         event_type: &'a str,
         body: Raw<AnyMessageEventContent>,
     ) -> Self {
diff --git a/crates/ruma-client-api/src/r0/redact/redact_event.rs b/crates/ruma-client-api/src/r0/redact/redact_event.rs
index a0546793..448829c7 100644
--- a/crates/ruma-client-api/src/r0/redact/redact_event.rs
+++ b/crates/ruma-client-api/src/r0/redact/redact_event.rs
@@ -1,7 +1,7 @@
 //! [PUT /_matrix/client/r0/rooms/{roomId}/redact/{eventId}/{txnId}](https://matrix.org/docs/spec/client_server/r0.6.1#put-matrix-client-r0-rooms-roomid-redact-eventid-txnid)
 
 use ruma_api::ruma_api;
-use ruma_identifiers::{EventId, RoomId};
+use ruma_identifiers::{EventId, RoomId, TransactionId};
 
 ruma_api! {
     metadata: {
@@ -27,7 +27,7 @@ ruma_api! {
         /// Clients should generate a unique ID; it will be used by the server to ensure idempotency
         /// of requests.
         #[ruma_api(path)]
-        pub txn_id: &'a str,
+        pub txn_id: &'a TransactionId,
 
         /// The reason for the redaction.
         #[serde(skip_serializing_if = "Option::is_none")]
@@ -44,7 +44,7 @@ ruma_api! {
 
 impl<'a> Request<'a> {
     /// Creates a new `Request` with the given room ID, event ID and transaction ID.
-    pub fn new(room_id: &'a RoomId, event_id: &'a EventId, txn_id: &'a str) -> Self {
+    pub fn new(room_id: &'a RoomId, event_id: &'a EventId, txn_id: &'a TransactionId) -> Self {
         Self { room_id, event_id, txn_id, reason: None }
     }
 }
diff --git a/crates/ruma-client-api/src/r0/to_device/send_event_to_device.rs b/crates/ruma-client-api/src/r0/to_device/send_event_to_device.rs
index e8d959e0..aa08e207 100644
--- a/crates/ruma-client-api/src/r0/to_device/send_event_to_device.rs
+++ b/crates/ruma-client-api/src/r0/to_device/send_event_to_device.rs
@@ -5,7 +5,7 @@ use std::collections::BTreeMap;
 use ruma_api::ruma_api;
 use ruma_common::to_device::DeviceIdOrAllDevices;
 use ruma_events::AnyToDeviceEventContent;
-use ruma_identifiers::UserId;
+use ruma_identifiers::{TransactionId, UserId};
 use ruma_serde::Raw;
 
 ruma_api! {
@@ -25,7 +25,7 @@ ruma_api! {
 
         /// A request identifier unique to the access token used to send the request.
         #[ruma_api(path)]
-        pub txn_id: &'a str,
+        pub txn_id: &'a TransactionId,
 
         /// Messages to send.
         ///
@@ -42,7 +42,7 @@ ruma_api! {
 
 impl<'a> Request<'a> {
     /// Creates a new `Request` with the given event type, transaction ID and raw messages.
-    pub fn new_raw(event_type: &'a str, txn_id: &'a str, messages: Messages) -> Self {
+    pub fn new_raw(event_type: &'a str, txn_id: &'a TransactionId, messages: Messages) -> Self {
         Self { event_type, txn_id, messages }
     }
 }
diff --git a/crates/ruma-client-api/src/r0/uiaa.rs b/crates/ruma-client-api/src/r0/uiaa.rs
index 0be71a79..8ec95dd7 100644
--- a/crates/ruma-client-api/src/r0/uiaa.rs
+++ b/crates/ruma-client-api/src/r0/uiaa.rs
@@ -10,7 +10,7 @@ use ruma_api::{
     EndpointError, OutgoingResponse,
 };
 use ruma_common::thirdparty::Medium;
-use ruma_identifiers::{ClientSecret, SessionId};
+use ruma_identifiers::{ClientSecret, SessionId, TransactionId};
 use ruma_serde::{JsonObject, Outgoing, StringEnum};
 use serde::{
     de::{self, DeserializeOwned},
@@ -470,7 +470,7 @@ pub struct Token<'a> {
     pub token: &'a str,
 
     /// The transaction ID.
-    pub txn_id: &'a str,
+    pub txn_id: &'a TransactionId,
 
     /// The value of the session key given by the homeserver, if any.
     pub session: Option<&'a str>,
@@ -478,7 +478,7 @@ pub struct Token<'a> {
 
 impl<'a> Token<'a> {
     /// Creates a new `Token` with the given token and transaction ID.
-    pub fn new(token: &'a str, txn_id: &'a str) -> Self {
+    pub fn new(token: &'a str, txn_id: &'a TransactionId) -> Self {
         Self { token, txn_id, session: None }
     }
 }
diff --git a/crates/ruma-client-api/tests/uiaa.rs b/crates/ruma-client-api/tests/uiaa.rs
index c77bb8ee..216d231e 100644
--- a/crates/ruma-client-api/tests/uiaa.rs
+++ b/crates/ruma-client-api/tests/uiaa.rs
@@ -30,7 +30,7 @@ fn deserialize_user_identifier() {
 #[test]
 fn serialize_auth_data_token() {
     let auth_data = AuthData::Token(
-        assign!(uiaa::Token::new("mytoken", "txn123"), { session: Some("session") }),
+        assign!(uiaa::Token::new("mytoken", "txn123".into()), { session: Some("session") }),
     );
 
     assert_matches!(
diff --git a/crates/ruma-events/src/key/verification/accept.rs b/crates/ruma-events/src/key/verification/accept.rs
index 7082f7f9..48cd4761 100644
--- a/crates/ruma-events/src/key/verification/accept.rs
+++ b/crates/ruma-events/src/key/verification/accept.rs
@@ -5,6 +5,7 @@
 use std::collections::BTreeMap;
 
 use ruma_events_macros::EventContent;
+use ruma_identifiers::TransactionId;
 use ruma_serde::Base64;
 use serde::{Deserialize, Serialize};
 use serde_json::Value as JsonValue;
@@ -25,7 +26,7 @@ pub struct ToDeviceKeyVerificationAcceptEventContent {
     /// An opaque identifier for the verification process.
     ///
     /// Must be the same as the one used for the `m.key.verification.start` message.
-    pub transaction_id: String,
+    pub transaction_id: Box<TransactionId>,
 
     /// The method specific content.
     #[serde(flatten)]
@@ -35,7 +36,7 @@ pub struct ToDeviceKeyVerificationAcceptEventContent {
 impl ToDeviceKeyVerificationAcceptEventContent {
     /// Creates a new `ToDeviceKeyVerificationAcceptEventContent` with the given transaction ID and
     /// method-specific content.
-    pub fn new(transaction_id: String, method: AcceptMethod) -> Self {
+    pub fn new(transaction_id: Box<TransactionId>, method: AcceptMethod) -> Self {
         Self { transaction_id, method }
     }
 }
diff --git a/crates/ruma-events/src/key/verification/cancel.rs b/crates/ruma-events/src/key/verification/cancel.rs
index ecf5ec1b..a828898e 100644
--- a/crates/ruma-events/src/key/verification/cancel.rs
+++ b/crates/ruma-events/src/key/verification/cancel.rs
@@ -3,6 +3,7 @@
 //! [`m.key.verification.cancel`]: https://spec.matrix.org/v1.1/client-server-api/#mkeyverificationcancel
 
 use ruma_events_macros::EventContent;
+use ruma_identifiers::TransactionId;
 use ruma_serde::StringEnum;
 use serde::{Deserialize, Serialize};
 
@@ -19,7 +20,7 @@ use super::Relation;
 #[ruma_event(type = "m.key.verification.cancel", kind = ToDevice)]
 pub struct ToDeviceKeyVerificationCancelEventContent {
     /// The opaque identifier for the verification process/request.
-    pub transaction_id: String,
+    pub transaction_id: Box<TransactionId>,
 
     /// A human readable description of the `code`.
     ///
@@ -33,7 +34,7 @@ pub struct ToDeviceKeyVerificationCancelEventContent {
 impl ToDeviceKeyVerificationCancelEventContent {
     /// Creates a new `ToDeviceKeyVerificationCancelEventContent` with the given transaction ID,
     /// reason and code.
-    pub fn new(transaction_id: String, reason: String, code: CancelCode) -> Self {
+    pub fn new(transaction_id: Box<TransactionId>, reason: String, code: CancelCode) -> Self {
         Self { transaction_id, reason, code }
     }
 }
diff --git a/crates/ruma-events/src/key/verification/done.rs b/crates/ruma-events/src/key/verification/done.rs
index c8e97d21..e188bc46 100644
--- a/crates/ruma-events/src/key/verification/done.rs
+++ b/crates/ruma-events/src/key/verification/done.rs
@@ -3,6 +3,7 @@
 //! [`m.key.verification.done`]: https://spec.matrix.org/v1.1/client-server-api/#mkeyverificationdone
 
 use ruma_events_macros::EventContent;
+use ruma_identifiers::TransactionId;
 use serde::{Deserialize, Serialize};
 
 use super::Relation;
@@ -17,12 +18,12 @@ pub struct ToDeviceKeyVerificationDoneEventContent {
     /// An opaque identifier for the verification process.
     ///
     /// Must be the same as the one used for the `m.key.verification.start` message.
-    pub transaction_id: String,
+    pub transaction_id: Box<TransactionId>,
 }
 
 impl ToDeviceKeyVerificationDoneEventContent {
     /// Creates a new `ToDeviceKeyVerificationDoneEventContent` with the given transaction ID.
-    pub fn new(transaction_id: String) -> Self {
+    pub fn new(transaction_id: Box<TransactionId>) -> Self {
         Self { transaction_id }
     }
 }
diff --git a/crates/ruma-events/src/key/verification/key.rs b/crates/ruma-events/src/key/verification/key.rs
index 8d31bd69..3e5cd4b7 100644
--- a/crates/ruma-events/src/key/verification/key.rs
+++ b/crates/ruma-events/src/key/verification/key.rs
@@ -3,6 +3,7 @@
 //! [`m.key.verification.key`]: https://spec.matrix.org/v1.1/client-server-api/#mkeyverificationkey
 
 use ruma_events_macros::EventContent;
+use ruma_identifiers::TransactionId;
 use ruma_serde::Base64;
 use serde::{Deserialize, Serialize};
 
@@ -19,7 +20,7 @@ pub struct ToDeviceKeyVerificationKeyEventContent {
     /// An opaque identifier for the verification process.
     ///
     /// Must be the same as the one used for the `m.key.verification.start` message.
-    pub transaction_id: String,
+    pub transaction_id: Box<TransactionId>,
 
     /// The device's ephemeral public key, encoded as unpadded base64.
     pub key: Base64,
@@ -28,7 +29,7 @@ pub struct ToDeviceKeyVerificationKeyEventContent {
 impl ToDeviceKeyVerificationKeyEventContent {
     /// Creates a new `ToDeviceKeyVerificationKeyEventContent` with the given transaction ID and
     /// key.
-    pub fn new(transaction_id: String, key: Base64) -> Self {
+    pub fn new(transaction_id: Box<TransactionId>, key: Base64) -> Self {
         Self { transaction_id, key }
     }
 }
diff --git a/crates/ruma-events/src/key/verification/mac.rs b/crates/ruma-events/src/key/verification/mac.rs
index 4f57892b..64d449cc 100644
--- a/crates/ruma-events/src/key/verification/mac.rs
+++ b/crates/ruma-events/src/key/verification/mac.rs
@@ -5,6 +5,7 @@
 use std::collections::BTreeMap;
 
 use ruma_events_macros::EventContent;
+use ruma_identifiers::TransactionId;
 use ruma_serde::Base64;
 use serde::{Deserialize, Serialize};
 
@@ -21,7 +22,7 @@ pub struct ToDeviceKeyVerificationMacEventContent {
     /// An opaque identifier for the verification process.
     ///
     /// Must be the same as the one used for the `m.key.verification.start` message.
-    pub transaction_id: String,
+    pub transaction_id: Box<TransactionId>,
 
     /// A map of the key ID to the MAC of the key, using the algorithm in the verification process.
     ///
@@ -36,7 +37,11 @@ pub struct ToDeviceKeyVerificationMacEventContent {
 impl ToDeviceKeyVerificationMacEventContent {
     /// Creates a new `ToDeviceKeyVerificationMacEventContent` with the given transaction ID, key ID
     /// to MAC map and key MAC.
-    pub fn new(transaction_id: String, mac: BTreeMap<String, Base64>, keys: Base64) -> Self {
+    pub fn new(
+        transaction_id: Box<TransactionId>,
+        mac: BTreeMap<String, Base64>,
+        keys: Base64,
+    ) -> Self {
         Self { transaction_id, mac, keys }
     }
 }
diff --git a/crates/ruma-events/src/key/verification/ready.rs b/crates/ruma-events/src/key/verification/ready.rs
index e34c902a..edf49d3a 100644
--- a/crates/ruma-events/src/key/verification/ready.rs
+++ b/crates/ruma-events/src/key/verification/ready.rs
@@ -3,7 +3,7 @@
 //! [`m.key.verification.ready`]: https://spec.matrix.org/v1.1/client-server-api/#mkeyverificationready
 
 use ruma_events_macros::EventContent;
-use ruma_identifiers::DeviceId;
+use ruma_identifiers::{DeviceId, TransactionId};
 use serde::{Deserialize, Serialize};
 
 use super::{Relation, VerificationMethod};
@@ -26,7 +26,7 @@ pub struct ToDeviceKeyVerificationReadyEventContent {
     /// Must be unique with respect to the devices involved. Must be the same as the
     /// `transaction_id` given in the `m.key.verification.request` from a
     /// request.
-    pub transaction_id: String,
+    pub transaction_id: Box<TransactionId>,
 }
 
 impl ToDeviceKeyVerificationReadyEventContent {
@@ -35,7 +35,7 @@ impl ToDeviceKeyVerificationReadyEventContent {
     pub fn new(
         from_device: Box<DeviceId>,
         methods: Vec<VerificationMethod>,
-        transaction_id: String,
+        transaction_id: Box<TransactionId>,
     ) -> Self {
         Self { from_device, methods, transaction_id }
     }
@@ -111,7 +111,7 @@ mod tests {
 
         let content = ToDeviceKeyVerificationReadyEventContent {
             from_device: device,
-            transaction_id: "456".to_owned(),
+            transaction_id: "456".into(),
             methods: vec![VerificationMethod::SasV1],
         };
 
diff --git a/crates/ruma-events/src/key/verification/request.rs b/crates/ruma-events/src/key/verification/request.rs
index aa388c50..4ab6742f 100644
--- a/crates/ruma-events/src/key/verification/request.rs
+++ b/crates/ruma-events/src/key/verification/request.rs
@@ -4,7 +4,7 @@
 
 use ruma_common::MilliSecondsSinceUnixEpoch;
 use ruma_events_macros::EventContent;
-use ruma_identifiers::DeviceId;
+use ruma_identifiers::{DeviceId, TransactionId};
 use serde::{Deserialize, Serialize};
 
 use super::VerificationMethod;
@@ -20,7 +20,7 @@ pub struct ToDeviceKeyVerificationRequestEventContent {
     /// An opaque identifier for the verification request.
     ///
     /// Must be unique with respect to the devices involved.
-    pub transaction_id: String,
+    pub transaction_id: Box<TransactionId>,
 
     /// The verification methods supported by the sender.
     pub methods: Vec<VerificationMethod>,
@@ -37,7 +37,7 @@ impl ToDeviceKeyVerificationRequestEventContent {
     /// transaction ID, methods and timestamp.
     pub fn new(
         from_device: Box<DeviceId>,
-        transaction_id: String,
+        transaction_id: Box<TransactionId>,
         methods: Vec<VerificationMethod>,
         timestamp: MilliSecondsSinceUnixEpoch,
     ) -> Self {
diff --git a/crates/ruma-events/src/key/verification/start.rs b/crates/ruma-events/src/key/verification/start.rs
index 07c784a4..b54d4712 100644
--- a/crates/ruma-events/src/key/verification/start.rs
+++ b/crates/ruma-events/src/key/verification/start.rs
@@ -5,7 +5,7 @@
 use std::collections::BTreeMap;
 
 use ruma_events_macros::EventContent;
-use ruma_identifiers::DeviceId;
+use ruma_identifiers::{DeviceId, TransactionId};
 #[cfg(feature = "unstable-pre-spec")]
 use ruma_serde::Base64;
 use serde::{Deserialize, Serialize};
@@ -32,7 +32,7 @@ pub struct ToDeviceKeyVerificationStartEventContent {
     /// Must be unique with respect to the devices involved. Must be the same as the
     /// `transaction_id` given in the `m.key.verification.request` if this process is originating
     /// from a request.
-    pub transaction_id: String,
+    pub transaction_id: Box<TransactionId>,
 
     /// Method specific content.
     #[serde(flatten)]
@@ -42,7 +42,11 @@ pub struct ToDeviceKeyVerificationStartEventContent {
 impl ToDeviceKeyVerificationStartEventContent {
     /// Creates a new `ToDeviceKeyVerificationStartEventContent` with the given device ID,
     /// transaction ID and method specific content.
-    pub fn new(from_device: Box<DeviceId>, transaction_id: String, method: StartMethod) -> Self {
+    pub fn new(
+        from_device: Box<DeviceId>,
+        transaction_id: Box<TransactionId>,
+        method: StartMethod,
+    ) -> Self {
         Self { from_device, transaction_id, method }
     }
 }
diff --git a/crates/ruma-events/src/unsigned.rs b/crates/ruma-events/src/unsigned.rs
index 4de9872b..4622149d 100644
--- a/crates/ruma-events/src/unsigned.rs
+++ b/crates/ruma-events/src/unsigned.rs
@@ -1,4 +1,5 @@
 use js_int::Int;
+use ruma_identifiers::TransactionId;
 use serde::{Deserialize, Serialize};
 
 #[cfg(feature = "unstable-pre-spec")]
@@ -20,7 +21,7 @@ pub struct Unsigned {
     /// The client-supplied transaction ID, if the client being given the event is the same one
     /// which sent it.
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub transaction_id: Option<String>,
+    pub transaction_id: Option<Box<TransactionId>>,
 
     /// Server-compiled information from other events relating to this event.
     #[cfg(feature = "unstable-pre-spec")]
@@ -82,7 +83,7 @@ pub struct UnsignedWithPrevContent {
     age: Option<Int>,
 
     #[serde(skip_serializing_if = "Option::is_none")]
-    transaction_id: Option<String>,
+    transaction_id: Option<Box<TransactionId>>,
 
     #[cfg(feature = "unstable-pre-spec")]
     #[serde(rename = "m.relations", skip_serializing_if = "Option::is_none")]
diff --git a/crates/ruma-federation-api/src/transactions/send_transaction_message/v1.rs b/crates/ruma-federation-api/src/transactions/send_transaction_message/v1.rs
index 779126ea..f6876c3a 100644
--- a/crates/ruma-federation-api/src/transactions/send_transaction_message/v1.rs
+++ b/crates/ruma-federation-api/src/transactions/send_transaction_message/v1.rs
@@ -4,7 +4,7 @@ use std::collections::BTreeMap;
 
 use ruma_api::ruma_api;
 use ruma_common::MilliSecondsSinceUnixEpoch;
-use ruma_identifiers::{EventId, ServerName};
+use ruma_identifiers::{EventId, ServerName, TransactionId};
 use ruma_serde::Raw;
 use serde_json::value::RawValue as RawJsonValue;
 
@@ -23,7 +23,7 @@ ruma_api! {
     request: {
         /// A transaction ID unique between sending and receiving homeservers.
         #[ruma_api(path)]
-        pub transaction_id: &'a str,
+        pub transaction_id: &'a TransactionId,
 
         /// The server_name of the homeserver sending this transaction.
         pub origin: &'a ServerName,
@@ -58,7 +58,7 @@ impl<'a> Request<'a> {
     ///
     /// The PDU and EDU lists will start off empty.
     pub fn new(
-        transaction_id: &'a str,
+        transaction_id: &'a TransactionId,
         origin: &'a ServerName,
         origin_server_ts: MilliSecondsSinceUnixEpoch,
     ) -> Self {
diff --git a/crates/ruma-identifiers/Cargo.toml b/crates/ruma-identifiers/Cargo.toml
index 50fd5877..9b19e28c 100644
--- a/crates/ruma-identifiers/Cargo.toml
+++ b/crates/ruma-identifiers/Cargo.toml
@@ -21,19 +21,21 @@ rustdoc-args = ["--cfg", "docsrs"]
 [features]
 default = ["serde"]
 compat = ["ruma-identifiers-validation/compat"]
+rand = ["rand_crate", "uuid"]
 serde = ["ruma-serde", "serde1"]
 unstable-pre-spec = []
 
 [dependencies]
 either = { version = "1.6.1", optional = true }
 percent-encoding = "2.1.0"
-rand = { version = "0.8.3", optional = true }
+rand_crate = { package = "rand", version = "0.8.3", optional = true }
 ruma-identifiers-macros = { version = "=0.20.0", path = "../ruma-identifiers-macros" }
 ruma-identifiers-validation = { version = "0.5.0", path = "../ruma-identifiers-validation", default-features = false }
 ruma-serde = { version = "0.5.0", path = "../ruma-serde", optional = true }
 ruma-serde-macros = { version = "0.5.0", path = "../ruma-serde-macros" }
 # Renamed so we can have a serde feature.
 serde1 = { package = "serde", version = "1.0.126", optional = true, features = ["derive"] }
+uuid = { version = "0.8.2", optional = true, features = ["v4"] }
 
 [dev-dependencies]
 matches = "0.1.8"
diff --git a/crates/ruma-identifiers/src/lib.rs b/crates/ruma-identifiers/src/lib.rs
index 9614de57..1b839a83 100644
--- a/crates/ruma-identifiers/src/lib.rs
+++ b/crates/ruma-identifiers/src/lib.rs
@@ -8,11 +8,14 @@
 #![allow(unused_qualifications)]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 
-// Renamed in `Cargo.toml` so we can have a serde feature.
-// Rename it back here because `serde1` is ugly.
+// Renamed in `Cargo.toml` so we can features with the same name as the package.
+// Rename them back here because the `Cargo.toml` names are ugly.
 #[cfg(feature = "serde")]
 extern crate serde1 as serde;
 
+#[cfg(feature = "rand")]
+extern crate rand_crate as rand;
+
 #[cfg(feature = "serde")]
 use std::convert::TryFrom;
 
@@ -38,6 +41,7 @@ pub use crate::{
     server_name::ServerName,
     session_id::SessionId,
     signatures::{DeviceSignatures, EntitySignatures, ServerSignatures, Signatures},
+    transaction_id::TransactionId,
     user_id::UserId,
 };
 #[doc(inline)]
@@ -65,6 +69,7 @@ mod room_version_id;
 mod server_name;
 mod session_id;
 mod signatures;
+mod transaction_id;
 
 /// Generates a random identifier localpart.
 #[cfg(feature = "rand")]
diff --git a/crates/ruma-identifiers/src/transaction_id.rs b/crates/ruma-identifiers/src/transaction_id.rs
new file mode 100644
index 00000000..2815cd88
--- /dev/null
+++ b/crates/ruma-identifiers/src/transaction_id.rs
@@ -0,0 +1,25 @@
+/// A Matrix transaction ID.
+///
+/// Transaction IDs in Matrix are opaque strings. This type is provided simply for its semantic
+/// value.
+///
+/// You can create one from a string (using `.into()`) but the recommended way is to use
+/// `TransactionId::new()` to generate a random one. If that function is not available for you, you
+/// need to activate this crate's `rand` Cargo feature.
+#[repr(transparent)]
+#[derive(PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct TransactionId(str);
+
+impl TransactionId {
+    /// Creates a random transaction ID.
+    ///
+    /// This will currently be a UUID without hyphens, but no guarantees are made about the
+    /// structure of transaction IDs generated from this function.
+    #[cfg(feature = "rand")]
+    pub fn new() -> Box<Self> {
+        let id = uuid::Uuid::new_v4();
+        Self::from_owned(id.to_simple().to_string().into_boxed_str())
+    }
+}
+
+opaque_identifier!(TransactionId);
diff --git a/crates/ruma-serde-macros/src/outgoing.rs b/crates/ruma-serde-macros/src/outgoing.rs
index 3b0cccbf..00c2241b 100644
--- a/crates/ruma-serde-macros/src/outgoing.rs
+++ b/crates/ruma-serde-macros/src/outgoing.rs
@@ -275,6 +275,7 @@ fn strip_lifetimes(field_type: &mut Type) -> bool {
                         || last_seg.ident == "RoomName"
                         || last_seg.ident == "ServerSigningKeyId"
                         || last_seg.ident == "SigningKeyId"
+                        || last_seg.ident == "TransactionId"
                         || last_seg.ident == "UserId"
                     {
                         // The identifiers that need to be boxed `Box<T>` since they are DST's.
diff --git a/crates/ruma/Cargo.toml b/crates/ruma/Cargo.toml
index 42d26f3d..6301337b 100644
--- a/crates/ruma/Cargo.toml
+++ b/crates/ruma/Cargo.toml
@@ -149,7 +149,7 @@ tokio-stream = { version = "0.1.1", default-features = false }
 
 [[example]]
 name = "hello_world"
-required-features = ["client-api-c", "client-ext-client-api", "client-hyper-native-tls"]
+required-features = ["client-api-c", "client-ext-client-api", "client-hyper-native-tls", "rand"]
 
 [[example]]
 name = "hello_isahc"
diff --git a/crates/ruma/examples/hello_world.rs b/crates/ruma/examples/hello_world.rs
index 8960af8a..5ad63d63 100644
--- a/crates/ruma/examples/hello_world.rs
+++ b/crates/ruma/examples/hello_world.rs
@@ -5,6 +5,7 @@ use ruma::{
     events::room::message::RoomMessageEventContent,
     RoomAliasId,
 };
+use ruma_identifiers::TransactionId;
 
 type MatrixClient = ruma_client::Client<ruma_client::http_client::HyperNativeTls>;
 
@@ -22,7 +23,7 @@ async fn hello_world(
     client
         .send_request(send_message_event::Request::new(
             &room_id,
-            "1",
+            &TransactionId::new(),
             &RoomMessageEventContent::text_plain("Hello World!"),
         )?)
         .await?;
diff --git a/examples/joke_bot/src/main.rs b/examples/joke_bot/src/main.rs
index 86362d0a..18a2b19a 100644
--- a/examples/joke_bot/src/main.rs
+++ b/examples/joke_bot/src/main.rs
@@ -1,10 +1,4 @@
-use std::{
-    convert::TryInto,
-    error::Error,
-    io,
-    process::exit,
-    time::{Duration, SystemTime},
-};
+use std::{convert::TryInto, error::Error, io, process::exit, time::Duration};
 
 use ruma::{
     api::client::r0::{
@@ -16,6 +10,7 @@ use ruma::{
         room::message::{MessageType, RoomMessageEventContent},
         AnySyncMessageEvent, AnySyncRoomEvent,
     },
+    identifiers::TransactionId,
     presence::PresenceState,
     serde::Raw,
     RoomId, UserId,
@@ -154,7 +149,7 @@ async fn handle_messages(
                 };
                 let joke_content = RoomMessageEventContent::text_plain(joke);
 
-                let txn_id = generate_txn_id();
+                let txn_id = TransactionId::new();
                 let req = send_message_event::Request::new(room_id, &txn_id, &joke_content)?;
                 // Do nothing if we can't send the message.
                 let _ = matrix_client.send_request(req).await;
@@ -175,23 +170,12 @@ async fn handle_invitations(
     let greeting = "Hello! My name is Mr. Bot! I like to tell jokes. Like this one: ";
     let joke = get_joke(http_client).await.unwrap_or_else(|_| "err... never mind.".to_owned());
     let content = RoomMessageEventContent::text_plain(format!("{}\n{}", greeting, joke));
-    let txn_id = generate_txn_id();
+    let txn_id = TransactionId::new();
     let message = send_message_event::Request::new(room_id, &txn_id, &content)?;
     matrix_client.send_request(message).await?;
     Ok(())
 }
 
-// Each message needs a unique transaction ID, otherwise the server thinks that the message is
-// being retransmitted. We could use a random value, or a counter, but to avoid having to store
-// the state, we'll just use the current time as a transaction ID.
-fn generate_txn_id() -> String {
-    SystemTime::now()
-        .duration_since(SystemTime::UNIX_EPOCH)
-        .expect("current time is earlier than Unix epoch")
-        .as_millis()
-        .to_string()
-}
-
 async fn get_joke(client: &HttpClient) -> Result<String, Box<dyn Error>> {
     let uri = "https://v2.jokeapi.dev/joke/Programming,Pun,Misc?safe-mode&type=single"
         .parse::<hyper::Uri>()?;