diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index c6791fb912..94bdc15dcf 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -492,7 +492,7 @@ class EventCreationHandler:
             membership = builder.content.get("membership", None)
             target = UserID.from_string(builder.state_key)
 
-            if membership in {Membership.JOIN, Membership.INVITE}:
+            if membership in {Membership.JOIN, Membership.INVITE, Membership.KNOCK}:
                 # If event doesn't include a display name, add one.
                 profile = self.profile_handler
                 content = builder.content
diff --git a/synapse/rest/__init__.py b/synapse/rest/__init__.py
index 40f5c32db2..c44a38af5e 100644
--- a/synapse/rest/__init__.py
+++ b/synapse/rest/__init__.py
@@ -39,6 +39,7 @@ from synapse.rest.client.v2_alpha import (
     filter,
     groups,
     keys,
+    knock,
     notifications,
     openid,
     password_policy,
@@ -121,6 +122,7 @@ class ClientRestResource(JsonResource):
         account_validity.register_servlets(hs, client_resource)
         relations.register_servlets(hs, client_resource)
         password_policy.register_servlets(hs, client_resource)
+        knock.register_servlets(hs, client_resource)
 
         # moving to /_synapse/admin
         admin.register_servlets_for_client_rest_resource(hs, client_resource)
diff --git a/synapse/rest/client/v2_alpha/knock.py b/synapse/rest/client/v2_alpha/knock.py
new file mode 100644
index 0000000000..1814683120
--- /dev/null
+++ b/synapse/rest/client/v2_alpha/knock.py
@@ -0,0 +1,103 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 Sorunome
+# Copyright 2020 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 typing import TYPE_CHECKING, List, Optional, Tuple
+
+from twisted.web.server import Request
+
+from synapse.api.errors import SynapseError
+from synapse.http.servlet import RestServlet, parse_json_object_from_request
+from synapse.logging.opentracing import set_tag
+from synapse.rest.client.transactions import HttpTransactionCache
+from synapse.types import JsonDict, RoomAlias, RoomID
+
+if TYPE_CHECKING:
+    from synapse.app.homeserver import HomeServer
+
+from ._base import client_patterns
+
+logger = logging.getLogger(__name__)
+
+
+class TransactionRestServlet(RestServlet):
+    def __init__(self, hs: "HomeServer"):
+        super(TransactionRestServlet, self).__init__()
+        self.txns = HttpTransactionCache(hs)
+
+
+class KnockRoomAliasServlet(TransactionRestServlet):
+    """
+    POST /knock/{roomIdOrAlias}
+    """
+
+    PATTERNS = client_patterns("/knock/(?P<room_identifier>[^/]*)")
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
+        self.room_member_handler = hs.get_room_member_handler()
+        self.auth = hs.get_auth()
+
+    async def on_POST(
+        self, request: Request, room_identifier: str, txn_id: Optional[str] = None,
+    ) -> Tuple[int, JsonDict]:
+        requester = await self.auth.get_user_by_req(request)
+
+        content = parse_json_object_from_request(request)
+        event_content = None
+        if "reason" in content:
+            event_content = {"reason": content["reason"]}
+
+        if RoomID.is_valid(room_identifier):
+            room_id = room_identifier
+            try:
+                remote_room_hosts = [
+                    x.decode("ascii") for x in request.args[b"server_name"]
+                ]  # type: Optional[List[str]]
+            except Exception:
+                remote_room_hosts = None
+        elif RoomAlias.is_valid(room_identifier):
+            handler = self.room_member_handler
+            room_alias = RoomAlias.from_string(room_identifier)
+            room_id_obj, remote_room_hosts = await handler.lookup_room_alias(room_alias)
+            room_id = room_id_obj.to_string()
+        else:
+            raise SynapseError(
+                400, "%s was not legal room ID or room alias" % (room_identifier,)
+            )
+
+        await self.room_member_handler.update_membership(
+            requester=requester,
+            target=requester.user,
+            room_id=room_id,
+            action="knock",
+            txn_id=txn_id,
+            third_party_signed=None,
+            remote_room_hosts=remote_room_hosts,
+            content=event_content,
+        )
+
+        return 200, {"room_id": room_id}
+
+    def on_PUT(self, request: Request, room_identifier: str, txn_id: str):
+        set_tag("txn_id", txn_id)
+
+        return self.txns.fetch_or_execute_request(
+            request, self.on_POST, request, room_identifier, txn_id
+        )
+
+
+def register_servlets(hs, http_server):
+    KnockRoomAliasServlet(hs).register(http_server)