diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py

index 3e17038301..7b24b45ae5 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -16,10 +16,10 @@
 """ This module contains REST servlets to do with rooms: /rooms/<paths> """
 import logging
 import re
-from typing import TYPE_CHECKING, Optional, Tuple
+from typing import TYPE_CHECKING, Dict, List, Optional, Tuple
 from urllib import parse as urlparse
 
-from synapse.api.constants import EventTypes, Membership
+from synapse.api.constants import EventContentFields, EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -36,8 +36,8 @@ from synapse.http.servlet import (
     parse_boolean,
     parse_integer,
     parse_json_object_from_request,
-    parse_list_from_args,
     parse_string,
+    parse_strings_from_args,
 )
 from synapse.http.site import SynapseRequest
 from synapse.logging.opentracing import set_tag
@@ -266,6 +266,288 @@ class RoomSendEventRestServlet(TransactionRestServlet):
         )
 
 
+class RoomBatchSendEventRestServlet(TransactionRestServlet):
+    """
+    API endpoint which can insert a chunk of events historically back in time
+    next to the given `prev_event`.
+
+    `chunk_id` comes from `next_chunk_id `in the response of the batch send
+    endpoint and is derived from the "insertion" events added to each chunk.
+    It's not required for the first batch send.
+
+    `state_events_at_start` is used to define the historical state events
+    needed to auth the events like join events. These events will float
+    outside of the normal DAG as outlier's and won't be visible in the chat
+    history which also allows us to insert multiple chunks without having a bunch
+    of `@mxid joined the room` noise between each chunk.
+
+    `events` is chronological chunk/list of events you want to insert.
+    There is a reverse-chronological constraint on chunks so once you insert
+    some messages, you can only insert older ones after that.
+    tldr; Insert chunks from your most recent history -> oldest history.
+
+    POST /_matrix/client/unstable/org.matrix.msc2716/rooms/<roomID>/batch_send?prev_event=<eventID>&chunk_id=<chunkID>
+    {
+        "events": [ ... ],
+        "state_events_at_start": [ ... ]
+    }
+    """
+
+    PATTERNS = (
+        re.compile(
+            "^/_matrix/client/unstable/org.matrix.msc2716"
+            "/rooms/(?P<room_id>[^/]*)/batch_send$"
+        ),
+    )
+
+    def __init__(self, hs):
+        super().__init__(hs)
+        self.hs = hs
+        self.store = hs.get_datastore()
+        self.state_store = hs.get_storage().state
+        self.event_creation_handler = hs.get_event_creation_handler()
+        self.room_member_handler = hs.get_room_member_handler()
+        self.auth = hs.get_auth()
+
+    async def inherit_depth_from_prev_ids(self, prev_event_ids) -> int:
+        (
+            most_recent_prev_event_id,
+            most_recent_prev_event_depth,
+        ) = await self.store.get_max_depth_of(prev_event_ids)
+
+        # We want to insert the historical event after the `prev_event` but before the successor event
+        #
+        # We inherit depth from the successor event instead of the `prev_event`
+        # because events returned from `/messages` are first sorted by `topological_ordering`
+        # which is just the `depth` and then tie-break with `stream_ordering`.
+        #
+        # We mark these inserted historical events as "backfilled" which gives them a
+        # negative `stream_ordering`. If we use the same depth as the `prev_event`,
+        # then our historical event will tie-break and be sorted before the `prev_event`
+        # when it should come after.
+        #
+        # We want to use the successor event depth so they appear after `prev_event` because
+        # it has a larger `depth` but before the successor event because the `stream_ordering`
+        # is negative before the successor event.
+        successor_event_ids = await self.store.get_successor_events(
+            [most_recent_prev_event_id]
+        )
+
+        # If we can't find any successor events, then it's a forward extremity of
+        # historical messages and we can just inherit from the previous historical
+        # event which we can already assume has the correct depth where we want
+        # to insert into.
+        if not successor_event_ids:
+            depth = most_recent_prev_event_depth
+        else:
+            (
+                _,
+                oldest_successor_depth,
+            ) = await self.store.get_min_depth_of(successor_event_ids)
+
+            depth = oldest_successor_depth
+
+        return depth
+
+    async def on_POST(self, request, room_id):
+        requester = await self.auth.get_user_by_req(request, allow_guest=False)
+
+        if not requester.app_service:
+            raise AuthError(
+                403,
+                "Only application services can use the /batchsend endpoint",
+            )
+
+        body = parse_json_object_from_request(request)
+        assert_params_in_dict(body, ["state_events_at_start", "events"])
+
+        prev_events_from_query = parse_strings_from_args(request.args, "prev_event")
+        chunk_id_from_query = parse_string(request, "chunk_id", default=None)
+
+        if prev_events_from_query is None:
+            raise SynapseError(
+                400,
+                "prev_event query parameter is required when inserting historical messages back in time",
+                errcode=Codes.MISSING_PARAM,
+            )
+
+        # For the event we are inserting next to (`prev_events_from_query`),
+        # find the most recent auth events (derived from state events) that
+        # allowed that message to be sent. We will use that as a base
+        # to auth our historical messages against.
+        (
+            most_recent_prev_event_id,
+            _,
+        ) = await self.store.get_max_depth_of(prev_events_from_query)
+        # mapping from (type, state_key) -> state_event_id
+        prev_state_map = await self.state_store.get_state_ids_for_event(
+            most_recent_prev_event_id
+        )
+        # List of state event ID's
+        prev_state_ids = list(prev_state_map.values())
+        auth_event_ids = prev_state_ids
+
+        for state_event in body["state_events_at_start"]:
+            assert_params_in_dict(
+                state_event, ["type", "origin_server_ts", "content", "sender"]
+            )
+
+            logger.debug(
+                "RoomBatchSendEventRestServlet inserting state_event=%s, auth_event_ids=%s",
+                state_event,
+                auth_event_ids,
+            )
+
+            event_dict = {
+                "type": state_event["type"],
+                "origin_server_ts": state_event["origin_server_ts"],
+                "content": state_event["content"],
+                "room_id": room_id,
+                "sender": state_event["sender"],
+                "state_key": state_event["state_key"],
+            }
+
+            # Make the state events float off on their own
+            fake_prev_event_id = "$" + random_string(43)
+
+            # TODO: This is pretty much the same as some other code to handle inserting state in this file
+            if event_dict["type"] == EventTypes.Member:
+                membership = event_dict["content"].get("membership", None)
+                event_id, _ = await self.room_member_handler.update_membership(
+                    requester,
+                    target=UserID.from_string(event_dict["state_key"]),
+                    room_id=room_id,
+                    action=membership,
+                    content=event_dict["content"],
+                    outlier=True,
+                    prev_event_ids=[fake_prev_event_id],
+                    # Make sure to use a copy of this list because we modify it
+                    # later in the loop here. Otherwise it will be the same
+                    # reference and also update in the event when we append later.
+                    auth_event_ids=auth_event_ids.copy(),
+                )
+            else:
+                # TODO: Add some complement tests that adds state that is not member joins
+                # and will use this code path. Maybe we only want to support join state events
+                # and can get rid of this `else`?
+                (
+                    event,
+                    _,
+                ) = await self.event_creation_handler.create_and_send_nonmember_event(
+                    requester,
+                    event_dict,
+                    outlier=True,
+                    prev_event_ids=[fake_prev_event_id],
+                    # Make sure to use a copy of this list because we modify it
+                    # later in the loop here. Otherwise it will be the same
+                    # reference and also update in the event when we append later.
+                    auth_event_ids=auth_event_ids.copy(),
+                )
+                event_id = event.event_id
+
+            auth_event_ids.append(event_id)
+
+        events_to_create = body["events"]
+
+        # If provided, connect the chunk to the last insertion point
+        # The chunk ID passed in comes from the chunk_id in the
+        # "insertion" event from the previous chunk.
+        if chunk_id_from_query:
+            last_event_in_chunk = events_to_create[-1]
+            last_event_in_chunk["content"][
+                EventContentFields.MSC2716_CHUNK_ID
+            ] = chunk_id_from_query
+
+        # Add an "insertion" event to the start of each chunk (next to the oldest
+        # event in the chunk) so the next chunk can be connected to this one.
+        next_chunk_id = random_string(64)
+        insertion_event = {
+            "type": EventTypes.MSC2716_INSERTION,
+            "sender": requester.user.to_string(),
+            "content": {
+                EventContentFields.MSC2716_NEXT_CHUNK_ID: next_chunk_id,
+                EventContentFields.MSC2716_HISTORICAL: True,
+            },
+            # Since the insertion event is put at the start of the chunk,
+            # where the oldest event is, copy the origin_server_ts from
+            # the first event we're inserting
+            "origin_server_ts": events_to_create[0]["origin_server_ts"],
+        }
+        # Prepend the insertion event to the start of the chunk
+        events_to_create = [insertion_event] + events_to_create
+
+        inherited_depth = await self.inherit_depth_from_prev_ids(prev_events_from_query)
+
+        event_ids = []
+        prev_event_ids = prev_events_from_query
+        events_to_persist = []
+        for ev in events_to_create:
+            assert_params_in_dict(ev, ["type", "origin_server_ts", "content", "sender"])
+
+            # Mark all events as historical
+            # This has important semantics within the Synapse internals to backfill properly
+            ev["content"][EventContentFields.MSC2716_HISTORICAL] = True
+
+            event_dict = {
+                "type": ev["type"],
+                "origin_server_ts": ev["origin_server_ts"],
+                "content": ev["content"],
+                "room_id": room_id,
+                "sender": ev["sender"],  # requester.user.to_string(),
+                "prev_events": prev_event_ids.copy(),
+            }
+
+            event, context = await self.event_creation_handler.create_event(
+                requester,
+                event_dict,
+                prev_event_ids=event_dict.get("prev_events"),
+                auth_event_ids=auth_event_ids,
+                historical=True,
+                depth=inherited_depth,
+            )
+            logger.debug(
+                "RoomBatchSendEventRestServlet inserting event=%s, prev_event_ids=%s, auth_event_ids=%s",
+                event,
+                prev_event_ids,
+                auth_event_ids,
+            )
+
+            assert self.hs.is_mine_id(event.sender), "User must be our own: %s" % (
+                event.sender,
+            )
+
+            events_to_persist.append((event, context))
+            event_id = event.event_id
+
+            event_ids.append(event_id)
+            prev_event_ids = [event_id]
+
+        # Persist events in reverse-chronological order so they have the
+        # correct stream_ordering as they are backfilled (which decrements).
+        # Events are sorted by (topological_ordering, stream_ordering)
+        # where topological_ordering is just depth.
+        for (event, context) in reversed(events_to_persist):
+            ev = await self.event_creation_handler.handle_new_client_event(
+                requester=requester,
+                event=event,
+                context=context,
+            )
+
+        return 200, {
+            "state_events": auth_event_ids,
+            "events": event_ids,
+            "next_chunk_id": next_chunk_id,
+        }
+
+    def on_GET(self, request, room_id):
+        return 501, "Not implemented"
+
+    def on_PUT(self, request, room_id):
+        return self.txns.fetch_or_execute_request(
+            request, self.on_POST, request, room_id
+        )
+
+
 # TODO: Needs unit testing for room ID + alias joins
 class JoinRoomAliasServlet(TransactionRestServlet):
     def __init__(self, hs):
@@ -278,7 +560,12 @@ class JoinRoomAliasServlet(TransactionRestServlet):
         PATTERNS = "/join/(?P<room_identifier>[^/]*)"
         register_txn_path(self, PATTERNS, http_server)
 
-    async def on_POST(self, request, room_identifier, txn_id=None):
+    async def on_POST(
+        self,
+        request: SynapseRequest,
+        room_identifier: str,
+        txn_id: Optional[str] = None,
+    ):
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
         try:
@@ -290,15 +577,18 @@ class JoinRoomAliasServlet(TransactionRestServlet):
 
         if RoomID.is_valid(room_identifier):
             room_id = room_identifier
-            try:
-                remote_room_hosts = parse_list_from_args(request.args, "server_name")
-            except KeyError:
-                remote_room_hosts = None
+
+            # twisted.web.server.Request.args is incorrectly defined as Optional[Any]
+            args: Dict[bytes, List[bytes]] = request.args  # type: ignore
+
+            remote_room_hosts = parse_strings_from_args(
+                args, "server_name", required=False
+            )
         elif RoomAlias.is_valid(room_identifier):
             handler = self.room_member_handler
             room_alias = RoomAlias.from_string(room_identifier)
-            room_id, remote_room_hosts = await handler.lookup_room_alias(room_alias)
-            room_id = room_id.to_string()
+            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,)
@@ -1047,6 +1337,8 @@ class RoomSpaceSummaryRestServlet(RestServlet):
 
 
 def register_servlets(hs: "HomeServer", http_server, is_worker=False):
+    msc2716_enabled = hs.config.experimental.msc2716_enabled
+
     RoomStateEventRestServlet(hs).register(http_server)
     RoomMemberListRestServlet(hs).register(http_server)
     JoinedRoomMemberListRestServlet(hs).register(http_server)
@@ -1054,6 +1346,8 @@ def register_servlets(hs: "HomeServer", http_server, is_worker=False):
     JoinRoomAliasServlet(hs).register(http_server)
     RoomMembershipRestServlet(hs).register(http_server)
     RoomSendEventRestServlet(hs).register(http_server)
+    if msc2716_enabled:
+        RoomBatchSendEventRestServlet(hs).register(http_server)
     PublicRoomListRestServlet(hs).register(http_server)
     RoomStateRestServlet(hs).register(http_server)
     RoomRedactEventRestServlet(hs).register(http_server)