diff --git a/crates/storage-pg/.sqlx/query-f0b4af5a9d6f1cc707a935fd5f34526a54ebbed8eef8f885f3a6723bc8490908.json b/crates/storage-pg/.sqlx/query-f0b4af5a9d6f1cc707a935fd5f34526a54ebbed8eef8f885f3a6723bc8490908.json
new file mode 100644
index 000000000..1622b4062
--- /dev/null
+++ b/crates/storage-pg/.sqlx/query-f0b4af5a9d6f1cc707a935fd5f34526a54ebbed8eef8f885f3a6723bc8490908.json
@@ -0,0 +1,14 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "\n                DELETE FROM user_unsupported_third_party_ids\n                WHERE user_id = $1\n            ",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Left": [
+        "Uuid"
+      ]
+    },
+    "nullable": []
+  },
+  "hash": "f0b4af5a9d6f1cc707a935fd5f34526a54ebbed8eef8f885f3a6723bc8490908"
+}
diff --git a/crates/storage-pg/src/user/mod.rs b/crates/storage-pg/src/user/mod.rs
index bbf02567e..f63fce2c5 100644
--- a/crates/storage-pg/src/user/mod.rs
+++ b/crates/storage-pg/src/user/mod.rs
@@ -1,3 +1,4 @@
+// Copyright 2025, 2026 Element Creations Ltd.
 // Copyright 2024, 2025 New Vector Ltd.
 // Copyright 2021-2024 The Matrix.org Foundation C.I.C.
 //
@@ -434,6 +435,30 @@ impl UserRepository for PgUserRepository<'_> {
         Ok(user)
     }
 
+    #[tracing::instrument(
+        name = "db.user.delete_unsupported_threepids",
+        skip_all,
+        fields(
+            db.query.text,
+            %user.id,
+        ),
+        err,
+    )]
+    async fn delete_unsupported_threepids(&mut self, user: &User) -> Result<usize, Self::Error> {
+        let res = sqlx::query!(
+            r#"
+                DELETE FROM user_unsupported_third_party_ids
+                WHERE user_id = $1
+            "#,
+            Uuid::from(user.id),
+        )
+        .traced()
+        .execute(&mut *self.conn)
+        .await?;
+
+        Ok(res.rows_affected().try_into().unwrap_or(usize::MAX))
+    }
+
     #[tracing::instrument(
         name = "db.user.set_can_request_admin",
         skip_all,
diff --git a/crates/storage/src/user/mod.rs b/crates/storage/src/user/mod.rs
index 657909ef4..bfc0eaae1 100644
--- a/crates/storage/src/user/mod.rs
+++ b/crates/storage/src/user/mod.rs
@@ -1,3 +1,4 @@
+// Copyright 2025, 2026 Element Creations Ltd.
 // Copyright 2024, 2025 New Vector Ltd.
 // Copyright 2021-2024 The Matrix.org Foundation C.I.C.
 //
@@ -295,6 +296,24 @@ pub trait UserRepository: Send + Sync {
     /// Returns [`Self::Error`] if the underlying repository fails
     async fn reactivate(&mut self, user: User) -> Result<User, Self::Error>;
 
+    /// Delete all the unsupported third-party IDs of a [`User`].
+    ///
+    /// Those were imported by syn2mas and kept in case we wanted to support
+    /// them later. They still need to be cleaned up when a user deactivate
+    /// their account.
+    ///
+    /// Returns the number of deleted third-party IDs.
+    ///
+    /// # Parameters
+    ///
+    /// * `user`: The [`User`] whose unsupported third-party IDs should be
+    ///   deleted
+    ///
+    /// # Errors
+    ///
+    /// Returns [`Self::Error`] if the underlying repository fails
+    async fn delete_unsupported_threepids(&mut self, user: &User) -> Result<usize, Self::Error>;
+
     /// Set whether a [`User`] can request admin
     ///
     /// Returns the [`User`] with the new `can_request_admin` value
@@ -367,6 +386,7 @@ repository_impl!(UserRepository:
     async fn unlock(&mut self, user: User) -> Result<User, Self::Error>;
     async fn deactivate(&mut self, clock: &dyn Clock, user: User) -> Result<User, Self::Error>;
     async fn reactivate(&mut self, user: User) -> Result<User, Self::Error>;
+    async fn delete_unsupported_threepids(&mut self, user: &User) -> Result<usize, Self::Error>;
     async fn set_can_request_admin(
         &mut self,
         user: User,
diff --git a/crates/tasks/src/user.rs b/crates/tasks/src/user.rs
index e605670cc..73ea444a3 100644
--- a/crates/tasks/src/user.rs
+++ b/crates/tasks/src/user.rs
@@ -119,6 +119,17 @@ impl RunnableJob for DeactivateUserJob {
             .map_err(JobError::retry)?;
         info!(affected = n, "Removed all email addresses for user");
 
+        // Delete all unsupported third-party IDs for the user
+        let n = repo
+            .user()
+            .delete_unsupported_threepids(&user)
+            .await
+            .map_err(JobError::retry)?;
+        info!(
+            affected = n,
+            "Removed all unsupported third-party IDs for user"
+        );
+
         // Before calling back to the homeserver, commit the changes to the database, as
         // we want the user to be locked out as soon as possible
         repo.save().await.map_err(JobError::retry)?;