diff --git a/ruma-api/src/lib.rs b/ruma-api/src/lib.rs
index 5b1c0565..f9940611 100644
--- a/ruma-api/src/lib.rs
+++ b/ruma-api/src/lib.rs
@@ -25,7 +25,8 @@ use std::{
     error::Error as StdError,
 };
 
-use http::Method;
+use http::{uri::PathAndQuery, Method};
+use ruma_identifiers::UserId;
 
 /// Generates a `ruma_api::Endpoint` from a concise definition.
 ///
@@ -226,7 +227,7 @@ pub trait EndpointError: StdError + Sized + 'static {
 }
 
 /// A request type for a Matrix API endpoint, used for sending requests.
-pub trait OutgoingRequest {
+pub trait OutgoingRequest: Sized {
     /// A type capturing the expected error conditions the server can return.
     type EndpointError: EndpointError;
 
@@ -254,6 +255,44 @@ pub trait OutgoingRequest {
     ) -> Result<http::Request<Vec<u8>>, IntoHttpError>;
 }
 
+/// An extension to `OutgoingRequest` which provides Appservice specific methods
+pub trait OutgoingRequestAppserviceExt: OutgoingRequest {
+    /// Tries to convert this request into an `http::Request` and appends a virtual `user_id` to
+    /// [assert Appservice identity][id_assert].
+    ///
+    /// [id_assert]: https://matrix.org/docs/spec/application_service/r0.1.2#identity-assertion
+    fn try_into_http_request_with_user_id(
+        self,
+        base_url: &str,
+        access_token: Option<&str>,
+        user_id: UserId,
+    ) -> Result<http::Request<Vec<u8>>, IntoHttpError> {
+        let mut http_request = self.try_into_http_request(base_url, access_token)?;
+        let user_id_query =
+            ruma_serde::urlencoded::to_string(&[("user_id", &user_id.into_string())])?;
+
+        let uri = http_request.uri().to_owned();
+        let mut parts = uri.into_parts();
+
+        let path_and_query_with_user_id = match &parts.path_and_query {
+            Some(path_and_query) => match path_and_query.query() {
+                Some(_) => format!("{}&{}", path_and_query, user_id_query),
+                None => format!("{}?{}", path_and_query, user_id_query),
+            },
+            None => format!("/?{}", user_id_query),
+        };
+
+        parts.path_and_query =
+            Some(PathAndQuery::try_from(path_and_query_with_user_id).map_err(http::Error::from)?);
+
+        *http_request.uri_mut() = parts.try_into().map_err(http::Error::from)?;
+
+        Ok(http_request)
+    }
+}
+
+impl<T: OutgoingRequest> OutgoingRequestAppserviceExt for T {}
+
 /// A request type for a Matrix API endpoint, used for receiving requests.
 pub trait IncomingRequest: Sized {
     /// A type capturing the error conditions that can be returned in the response.
diff --git a/ruma-api/tests/conversions.rs b/ruma-api/tests/conversions.rs
index 69e6307f..2c6875eb 100644
--- a/ruma-api/tests/conversions.rs
+++ b/ruma-api/tests/conversions.rs
@@ -1,4 +1,6 @@
-use ruma_api::{ruma_api, IncomingRequest as _, OutgoingRequest as _};
+use ruma_api::{
+    ruma_api, IncomingRequest as _, OutgoingRequest as _, OutgoingRequestAppserviceExt as _,
+};
 use ruma_identifiers::{user_id, UserId};
 
 ruma_api! {
@@ -57,3 +59,85 @@ fn request_serde() -> Result<(), Box<dyn std::error::Error + 'static>> {
 
     Ok(())
 }
+
+#[test]
+fn request_with_user_id_serde() -> Result<(), Box<dyn std::error::Error + 'static>> {
+    let req = Request {
+        hello: "hi".to_owned(),
+        world: "test".to_owned(),
+        q1: "query_param_special_chars %/&@!".to_owned(),
+        q2: 55,
+        bar: "barVal".to_owned(),
+        baz: user_id!("@bazme:ruma.io"),
+    };
+
+    let user_id = user_id!("@_virtual_:ruma.io");
+    let http_req =
+        req.clone().try_into_http_request_with_user_id("https://homeserver.tld", None, user_id)?;
+
+    let query = http_req.uri().query().unwrap();
+
+    assert_eq!(
+        query,
+        "q1=query_param_special_chars+%25%2F%26%40%21&q2=55&user_id=%40_virtual_%3Aruma.io"
+    );
+
+    Ok(())
+}
+
+mod without_query {
+    use super::*;
+
+    ruma_api! {
+        metadata: {
+            description: "Does something without query.",
+            method: POST,
+            name: "my_endpoint",
+            path: "/_matrix/foo/:bar/:baz",
+            rate_limited: false,
+            authentication: None,
+        }
+
+        request: {
+            pub hello: String,
+            #[ruma_api(header = CONTENT_TYPE)]
+            pub world: String,
+            #[ruma_api(path)]
+            pub bar: String,
+            #[ruma_api(path)]
+            pub baz: UserId,
+        }
+
+        response: {
+            pub hello: String,
+            #[ruma_api(header = CONTENT_TYPE)]
+            pub world: String,
+            #[serde(skip_serializing_if = "Option::is_none")]
+            pub optional_flag: Option<bool>,
+        }
+    }
+
+    #[test]
+    fn request_without_query_with_user_id_serde() -> Result<(), Box<dyn std::error::Error + 'static>>
+    {
+        let req = Request {
+            hello: "hi".to_owned(),
+            world: "test".to_owned(),
+            bar: "barVal".to_owned(),
+            baz: user_id!("@bazme:ruma.io"),
+        };
+
+        let user_id = user_id!("@_virtual_:ruma.io");
+        let http_req = req.clone().try_into_http_request_with_user_id(
+            "https://homeserver.tld",
+            None,
+            user_id,
+        )?;
+
+        let query = http_req.uri().query().unwrap();
+
+        assert_eq!(query, "user_id=%40_virtual_%3Aruma.io");
+
+        Ok(())
+    }
+}