diff --git a/crates/ruma-client-api/Cargo.toml b/crates/ruma-client-api/Cargo.toml
index 28727702..bc122a56 100644
--- a/crates/ruma-client-api/Cargo.toml
+++ b/crates/ruma-client-api/Cargo.toml
@@ -18,6 +18,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 compat = []
 unstable-exhaustive-types = []
 unstable-msc2448 = []
+unstable-msc3440 = []
 unstable-msc3488 = []
 client = []
 server = []
diff --git a/crates/ruma-client-api/src/filter.rs b/crates/ruma-client-api/src/filter.rs
index 5fbfba5e..760c7b38 100644
--- a/crates/ruma-client-api/src/filter.rs
+++ b/crates/ruma-client-api/src/filter.rs
@@ -48,6 +48,40 @@ impl Default for EventFormat {
     }
 }
 
+/// Relation types as defined in `rel_type` of an `m.relates_to` field.
+///
+/// This type can hold an arbitrary string. To check for formats that are not available as a
+/// documented variant here, use its string representation, obtained through `.as_str()`.
+#[derive(Clone, Debug, PartialEq, Eq, StringEnum)]
+#[cfg(feature = "unstable-msc3440")]
+#[non_exhaustive]
+pub enum RelationType {
+    /// `m.annotation`, an annotation, principally used by reactions.
+    #[cfg(feature = "unstable-msc2677")]
+    #[ruma_enum(rename = "m.annotation")]
+    Annotation,
+
+    /// `m.replace`, a replacement.
+    #[cfg(feature = "unstable-msc2676")]
+    #[ruma_enum(rename = "m.replace")]
+    Replacement,
+
+    /// `m.thread`, a participant to a thread.
+    #[ruma_enum(rename = "io.element.thread")]
+    Thread,
+
+    #[doc(hidden)]
+    _Custom(PrivOwnedStr),
+}
+
+#[cfg(feature = "unstable-msc3440")]
+impl RelationType {
+    /// Creates a string slice from this `RelationType`.
+    pub fn as_str(&self) -> &str {
+        self.as_ref()
+    }
+}
+
 /// Filters to be applied to room events.
 #[derive(Clone, Debug, Default, Outgoing, Serialize)]
 #[cfg_attr(not(feature = "unstable-exhaustive-types"), non_exhaustive)]
@@ -111,6 +145,30 @@ pub struct RoomEventFilter<'a> {
     /// Defaults to `LazyLoadOptions::Disabled`.
     #[serde(flatten)]
     pub lazy_load_options: LazyLoadOptions,
+
+    /// A list of relation types to include.
+    ///
+    /// An event A is included in the filter only if there exists another event B which relates to
+    /// A with a `rel_type` which is defined in the list.
+    #[cfg(feature = "unstable-msc3440")]
+    #[serde(
+        rename = "io.element.relation_types",
+        default,
+        skip_serializing_if = "<[_]>::is_empty"
+    )]
+    pub related_by_rel_types: &'a [RelationType],
+
+    /// A list of senders to include.
+    ///
+    /// An event A is included in the filter only if there exists another event B which relates to
+    /// A, and which has a sender which is in the list.
+    #[cfg(feature = "unstable-msc3440")]
+    #[serde(
+        rename = "io.element.relation_senders",
+        default,
+        skip_serializing_if = "<[_]>::is_empty"
+    )]
+    pub related_by_senders: &'a [Box<UserId>],
 }
 
 impl<'a> RoomEventFilter<'a> {
@@ -128,7 +186,7 @@ impl<'a> RoomEventFilter<'a> {
 
     /// Returns `true` if all fields are empty.
     pub fn is_empty(&self) -> bool {
-        self.not_types.is_empty()
+        let empty = self.not_types.is_empty()
             && self.not_rooms.is_empty()
             && self.limit.is_none()
             && self.rooms.is_none()
@@ -136,14 +194,20 @@ impl<'a> RoomEventFilter<'a> {
             && self.senders.is_none()
             && self.types.is_none()
             && self.url_filter.is_none()
-            && self.lazy_load_options.is_disabled()
+            && self.lazy_load_options.is_disabled();
+
+        #[cfg(not(feature = "unstable-msc3440"))]
+        return empty;
+
+        #[cfg(feature = "unstable-msc3440")]
+        return empty && self.related_by_rel_types.is_empty() && self.related_by_senders.is_empty();
     }
 }
 
 impl IncomingRoomEventFilter {
     /// Returns `true` if all fields are empty.
     pub fn is_empty(&self) -> bool {
-        self.not_types.is_empty()
+        let empty = self.not_types.is_empty()
             && self.not_rooms.is_empty()
             && self.limit.is_none()
             && self.rooms.is_none()
@@ -151,7 +215,13 @@ impl IncomingRoomEventFilter {
             && self.senders.is_none()
             && self.types.is_none()
             && self.url_filter.is_none()
-            && self.lazy_load_options.is_disabled()
+            && self.lazy_load_options.is_disabled();
+
+        #[cfg(not(feature = "unstable-msc3440"))]
+        return empty;
+
+        #[cfg(feature = "unstable-msc3440")]
+        return empty && self.related_by_rel_types.is_empty() && self.related_by_senders.is_empty();
     }
 }
 
@@ -470,10 +540,26 @@ mod tests {
                 limit: None,
                 url_filter: Some(UrlFilter::EventsWithUrl),
                 lazy_load_options: LazyLoadOptions::Enabled { include_redundant_members: false },
-            } if types == vec!["m.room.message".to_owned()]
+                #[cfg(feature = "unstable-msc3440")]
+                related_by_rel_types,
+                #[cfg(feature = "unstable-msc3440")]
+                related_by_senders
+            } if {
+                let valid = types == vec!["m.room.message".to_owned()]
                 && not_types.is_empty()
                 && not_rooms.is_empty()
-                && not_senders.is_empty()
+                && not_senders.is_empty();
+
+                #[cfg(not(feature = "unstable-msc3440"))]
+                {
+                    valid
+                }
+
+                #[cfg(feature = "unstable-msc3440")]
+                {
+                    valid && related_by_rel_types.is_empty() && related_by_senders.is_empty()
+                }
+            }
         );
 
         Ok(())
diff --git a/crates/ruma/Cargo.toml b/crates/ruma/Cargo.toml
index cd082161..1f53acc2 100644
--- a/crates/ruma/Cargo.toml
+++ b/crates/ruma/Cargo.toml
@@ -118,7 +118,10 @@ unstable-msc2675 = ["ruma-common/unstable-msc2675"]
 unstable-msc2676 = ["ruma-common/unstable-msc2676"]
 unstable-msc2677 = ["ruma-common/unstable-msc2677"]
 unstable-msc3246 = ["ruma-common/unstable-msc3246"]
-unstable-msc3440 = ["ruma-common/unstable-msc3440"]
+unstable-msc3440 = [
+    "ruma-client-api/unstable-msc3440",
+    "ruma-common/unstable-msc3440",
+]
 unstable-msc3488 = [
     "ruma-client-api/unstable-msc3488",
     "ruma-common/unstable-msc3488",