diff --git a/changelog.d/17973.feature b/changelog.d/17973.feature
new file mode 100644
index 00000000000..261df6c72fe
--- /dev/null
+++ b/changelog.d/17973.feature
@@ -0,0 +1 @@
+Implement MSC4185: Event Visibility API.
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 3411179a2a3..75ac705d531 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -443,6 +443,9 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         # MSC4151: Report room API (Client-Server API)
         self.msc4151_enabled: bool = experimental.get("msc4151_enabled", False)
 
+        # MSC4185: Event Visibility API
+        self.msc4185_enabled: bool = experimental.get("msc4185_enabled", False)
+
         # MSC4210: Remove legacy mentions
         self.msc4210_enabled: bool = experimental.get("msc4210_enabled", False)
 
diff --git a/synapse/rest/__init__.py b/synapse/rest/__init__.py
index 4e594e6595f..f81aca7a892 100644
--- a/synapse/rest/__init__.py
+++ b/synapse/rest/__init__.py
@@ -27,6 +27,7 @@
     account,
     account_data,
     account_validity,
+    appservice_event_visibility,
     appservice_ping,
     auth,
     auth_issuer,
@@ -117,6 +118,7 @@
     password_policy.register_servlets,
     knock.register_servlets,
     appservice_ping.register_servlets,
+    appservice_event_visibility.register_servlets,
     admin.register_servlets_for_client_rest_resource,
     mutual_rooms.register_servlets,
     login_token_request.register_servlets,
diff --git a/synapse/rest/client/appservice_event_visibility.py b/synapse/rest/client/appservice_event_visibility.py
new file mode 100644
index 00000000000..37cb4581c01
--- /dev/null
+++ b/synapse/rest/client/appservice_event_visibility.py
@@ -0,0 +1,75 @@
+# Copyright 2024 The Matrix.org Foundation C.I.C
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Tuple
+
+from synapse.api.errors import (
+    Codes,
+    SynapseError,
+)
+from synapse.http.server import HttpServer
+from synapse.http.servlet import RestServlet
+from synapse.http.site import SynapseRequest
+from synapse.visibility import filter_events_for_client
+
+from ._base import client_patterns
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class AppserviceEventVisibilityrestServlet(RestServlet):
+    PATTERNS = client_patterns(
+        "/net.tadzik/appservice/(?P<appservice_id>[^/]*)/can_user_see_event/(?P<room_id>[^/]*)/(?P<user_id>[^/]*)/(?P<event_id>[^/]*)$",
+        unstable=True,
+        releases=(),
+    )
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        self.auth = hs.get_auth()
+        self.store = hs.get_datastores().main
+        self._storage_controllers = hs.get_storage_controllers()
+
+    async def on_GET(
+        self, request: SynapseRequest, appservice_id: str, room_id: str, user_id: str, event_id: str
+    ) -> Tuple[int, bool]:
+        logger.info('on_GET')
+        requester = await self.auth.get_user_by_req(request)
+
+        if not requester.app_service:
+            raise SynapseError(
+                HTTPStatus.FORBIDDEN,
+                "Only application services can use the /can_user_see_event endpoint",
+                Codes.FORBIDDEN,
+            )
+        elif requester.app_service.id != appservice_id:
+            raise SynapseError(
+                HTTPStatus.FORBIDDEN,
+                "Mismatching application service ID in path",
+                Codes.FORBIDDEN,
+            )
+
+        event = await self.store.get_event(event_id, check_room_id=room_id)
+        filtered = await filter_events_for_client(self._storage_controllers, user_id, [event])
+
+        return HTTPStatus.OK, bool(filtered)
+
+def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
+    if hs.config.experimental.msc4185_enabled:
+        AppserviceEventVisibilityrestServlet(hs).register(http_server)
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index ba1141bbe59..e46acd9bbb9 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -143,6 +143,8 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
                     "fi.mau.msc2815": self.config.experimental.msc2815_enabled,
                     # Adds a ping endpoint for appservices to check HS->AS connection
                     "fi.mau.msc2659.stable": True,  # TODO: remove when "v1.7" is added above
+                    # Adds Event visibility API
+                    "net.tadzik.msc4185": self.config.experimental.msc4185_enabled,
                     # TODO: this is no longer needed once unstable MSC3882 does not need to be supported:
                     "org.matrix.msc3882": self.config.auth.login_via_existing_enabled,
                     # Adds support for remotely enabling/disabling pushers, as per MSC3881