diff --git a/changelog.d/13810.feature b/changelog.d/13810.feature
new file mode 100644
index 0000000000..f0258af661
--- /dev/null
+++ b/changelog.d/13810.feature
@@ -0,0 +1 @@
+Add an admin API endpoint to find a user based on its external ID in an auth provider.
diff --git a/docs/admin_api/user_admin_api.md b/docs/admin_api/user_admin_api.md
index 975f05c929..3625c7b6c5 100644
--- a/docs/admin_api/user_admin_api.md
+++ b/docs/admin_api/user_admin_api.md
@@ -1155,3 +1155,41 @@ GET /_synapse/admin/v1/username_available?username=$localpart
 
 The request and response format is the same as the
 [/_matrix/client/r0/register/available](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available) API.
+
+### Find a user based on their ID in an auth provider
+
+The API is:
+
+```
+GET /_synapse/admin/v1/auth_providers/$provider/users/$external_id
+```
+
+When a user matched the given ID for the given provider, an HTTP code `200` with a response body like the following is returned:
+
+```json
+{
+    "user_id": "@hello:example.org"
+}
+```
+
+**Parameters**
+
+The following parameters should be set in the URL:
+
+- `provider` - The ID of the authentication provider, as advertised by the [`GET /_matrix/client/v3/login`](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3login) API in the `m.login.sso` authentication method.
+- `external_id` - The user ID from the authentication provider. Usually corresponds to the `sub` claim for OIDC providers, or to the `uid` attestation for SAML2 providers.
+
+The `external_id` may have characters that are not URL-safe (typically `/`, `:` or `@`), so it is advised to URL-encode those parameters.
+
+**Errors**
+
+Returns a `404` HTTP status code if no user was found, with a response body like this:
+
+```json
+{
+    "errcode":"M_NOT_FOUND",
+    "error":"User not found"
+}
+```
+
+_Added in Synapse 1.68.0._
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index bac754e1b1..885669f9c7 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -80,6 +80,7 @@ from synapse.rest.admin.users import (
     SearchUsersRestServlet,
     ShadowBanRestServlet,
     UserAdminServlet,
+    UserByExternalId,
     UserMembershipRestServlet,
     UserRegisterServlet,
     UserRestServletV2,
@@ -275,6 +276,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ListDestinationsRestServlet(hs).register(http_server)
     RoomMessagesRestServlet(hs).register(http_server)
     RoomTimestampToEventRestServlet(hs).register(http_server)
+    UserByExternalId(hs).register(http_server)
 
     # Some servlets only get registered for the main process.
     if hs.config.worker.worker_app is None:
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 78ee9b6532..2ca6b2d08a 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -1156,3 +1156,30 @@ class AccountDataRestServlet(RestServlet):
                 "rooms": by_room_data,
             },
         }
+
+
+class UserByExternalId(RestServlet):
+    """Find a user based on an external ID from an auth provider"""
+
+    PATTERNS = admin_patterns(
+        "/auth_providers/(?P<provider>[^/]*)/users/(?P<external_id>[^/]*)"
+    )
+
+    def __init__(self, hs: "HomeServer"):
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastores().main
+
+    async def on_GET(
+        self,
+        request: SynapseRequest,
+        provider: str,
+        external_id: str,
+    ) -> Tuple[int, JsonDict]:
+        await assert_requester_is_admin(self._auth, request)
+
+        user_id = await self._store.get_user_by_external_id(provider, external_id)
+
+        if user_id is None:
+            raise NotFoundError("User not found")
+
+        return HTTPStatus.OK, {"user_id": user_id}
diff --git a/tests/rest/admin/test_user.py b/tests/rest/admin/test_user.py
index ec5ccf6fca..9f536ceeb3 100644
--- a/tests/rest/admin/test_user.py
+++ b/tests/rest/admin/test_user.py
@@ -4140,3 +4140,90 @@ class AccountDataTestCase(unittest.HomeserverTestCase):
             {"b": 2},
             channel.json_body["account_data"]["rooms"]["test_room"]["m.per_room"],
         )
+
+
+class UsersByExternalIdTestCase(unittest.HomeserverTestCase):
+
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+    ]
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.store = hs.get_datastores().main
+
+        self.admin_user = self.register_user("admin", "pass", admin=True)
+        self.admin_user_tok = self.login("admin", "pass")
+
+        self.other_user = self.register_user("user", "pass")
+        self.get_success(
+            self.store.record_user_external_id(
+                "the-auth-provider", "the-external-id", self.other_user
+            )
+        )
+        self.get_success(
+            self.store.record_user_external_id(
+                "another-auth-provider", "a:complex@external/id", self.other_user
+            )
+        )
+
+    def test_no_auth(self) -> None:
+        """Try to lookup a user without authentication."""
+        url = (
+            "/_synapse/admin/v1/auth_providers/the-auth-provider/users/the-external-id"
+        )
+
+        channel = self.make_request(
+            "GET",
+            url,
+        )
+
+        self.assertEqual(401, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.MISSING_TOKEN, channel.json_body["errcode"])
+
+    def test_binding_does_not_exist(self) -> None:
+        """Tests that a lookup for an external ID that does not exist returns a 404"""
+        url = "/_synapse/admin/v1/auth_providers/the-auth-provider/users/unknown-id"
+
+        channel = self.make_request(
+            "GET",
+            url,
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(404, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.NOT_FOUND, channel.json_body["errcode"])
+
+    def test_success(self) -> None:
+        """Tests a successful external ID lookup"""
+        url = (
+            "/_synapse/admin/v1/auth_providers/the-auth-provider/users/the-external-id"
+        )
+
+        channel = self.make_request(
+            "GET",
+            url,
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertEqual(
+            {"user_id": self.other_user},
+            channel.json_body,
+        )
+
+    def test_success_urlencoded(self) -> None:
+        """Tests a successful external ID lookup with an url-encoded ID"""
+        url = "/_synapse/admin/v1/auth_providers/another-auth-provider/users/a%3Acomplex%40external%2Fid"
+
+        channel = self.make_request(
+            "GET",
+            url,
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertEqual(
+            {"user_id": self.other_user},
+            channel.json_body,
+        )