diff --git a/crates/matrix-synapse/src/lib.rs b/crates/matrix-synapse/src/lib.rs
index fb3b5f08..54575700 100644
--- a/crates/matrix-synapse/src/lib.rs
+++ b/crates/matrix-synapse/src/lib.rs
@@ -175,6 +175,37 @@ impl HomeserverConnection for SynapseConnection {
         })
     }
 
+    #[tracing::instrument(
+        name = "homeserver.is_localpart_available",
+        skip_all,
+        fields(
+            matrix.homeserver = self.homeserver,
+            matrix.localpart = localpart,
+        ),
+        err(Display),
+    )]
+    async fn is_localpart_available(&self, localpart: &str) -> Result<bool, Self::Error> {
+        let mut client = self
+            .http_client_factory
+            .client("homeserver.is_localpart_available");
+
+        let request = self
+            .get(&format!(
+                "_synapse/admin/v1/username_available?username={localpart}"
+            ))
+            .body(EmptyBody::new())?;
+
+        let response = client.ready().await?.call(request).await?;
+
+        match response.status() {
+            StatusCode::OK => Ok(true),
+            StatusCode::BAD_REQUEST => Ok(false),
+            _ => Err(anyhow::anyhow!(
+                "Failed to query localpart availability from Synapse"
+            )),
+        }
+    }
+
     #[tracing::instrument(
         name = "homeserver.provision_user",
         skip_all,
diff --git a/crates/matrix/src/lib.rs b/crates/matrix/src/lib.rs
index 6953ef3d..5e1589e3 100644
--- a/crates/matrix/src/lib.rs
+++ b/crates/matrix/src/lib.rs
@@ -219,6 +219,17 @@ pub trait HomeserverConnection: Send + Sync {
     /// be provisioned.
     async fn provision_user(&self, request: &ProvisionRequest) -> Result<bool, Self::Error>;
 
+    /// Check whether a given username is available on the homeserver.
+    ///
+    /// # Parameters
+    ///
+    /// * `localpart` - The localpart to check.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the homeserver is unreachable.
+    async fn is_localpart_available(&self, localpart: &str) -> Result<bool, Self::Error>;
+
     /// Create a device for a user on the homeserver.
     ///
     /// # Parameters
@@ -312,6 +323,10 @@ impl<T: HomeserverConnection + Send + Sync + ?Sized> HomeserverConnection for &T
         (**self).provision_user(request).await
     }
 
+    async fn is_localpart_available(&self, localpart: &str) -> Result<bool, Self::Error> {
+        (**self).is_localpart_available(localpart).await
+    }
+
     async fn create_device(&self, mxid: &str, device_id: &str) -> Result<(), Self::Error> {
         (**self).create_device(mxid, device_id).await
     }
diff --git a/crates/matrix/src/mock.rs b/crates/matrix/src/mock.rs
index d6e9ffe5..7a67c550 100644
--- a/crates/matrix/src/mock.rs
+++ b/crates/matrix/src/mock.rs
@@ -34,6 +34,7 @@ struct MockUser {
 pub struct HomeserverConnection {
     homeserver: String,
     users: RwLock<HashMap<String, MockUser>>,
+    reserved_localparts: RwLock<HashSet<&'static str>>,
 }
 
 impl HomeserverConnection {
@@ -45,8 +46,13 @@ impl HomeserverConnection {
         Self {
             homeserver: homeserver.into(),
             users: RwLock::new(HashMap::new()),
+            reserved_localparts: RwLock::new(HashSet::new()),
         }
     }
+
+    pub async fn reserve_localpart(&self, localpart: &'static str) {
+        self.reserved_localparts.write().await.insert(localpart);
+    }
 }
 
 #[async_trait]
@@ -98,6 +104,16 @@ impl crate::HomeserverConnection for HomeserverConnection {
         Ok(inserted)
     }
 
+    async fn is_localpart_available(&self, localpart: &str) -> Result<bool, Self::Error> {
+        if self.reserved_localparts.read().await.contains(localpart) {
+            return Ok(false);
+        }
+
+        let mxid = self.mxid(localpart);
+        let users = self.users.read().await;
+        Ok(!users.contains_key(&mxid))
+    }
+
     async fn create_device(&self, mxid: &str, device_id: &str) -> Result<(), Self::Error> {
         let mut users = self.users.write().await;
         let user = users.get_mut(mxid).context("User not found")?;
@@ -200,5 +216,14 @@ mod tests {
         // XXX: there is no API to query devices yet in the trait
         // Delete the device
         assert!(conn.delete_device(mxid, device).await.is_ok());
+
+        // The user we just created should be not available
+        assert!(!conn.is_localpart_available("test").await.unwrap());
+        // But another user should be
+        assert!(conn.is_localpart_available("alice").await.unwrap());
+
+        // Reserve the localpart, it should not be available anymore
+        conn.reserve_localpart("alice").await;
+        assert!(!conn.is_localpart_available("alice").await.unwrap());
     }
 }