diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index a436f6a65..9c63c272d 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -600,3 +600,6 @@ def read_config(
             "beeper_user_notification_counts_enabled",
             False,
         )
+
+        # MSC4446: Allow moving the fully read marker backwards.
+        self.msc4446_enabled: bool = experimental.get("msc4446_enabled", False)
diff --git a/synapse/handlers/read_marker.py b/synapse/handlers/read_marker.py
index 1df418d3b..9aeac3ee1 100644
--- a/synapse/handlers/read_marker.py
+++ b/synapse/handlers/read_marker.py
@@ -44,6 +44,7 @@ async def received_client_read_marker(
         room_id: str,
         user_id: str,
         event_id: str,
+        allow_backward: bool = False,
         extra_content: Optional[JsonDict] = None,
     ) -> None:
         """Updates the read marker for a given user in a given room if the event ID given
@@ -62,7 +63,7 @@ async def received_client_read_marker(
             # Get event ordering, this also ensures we know about the event
             event_ordering = await self.store.get_event_ordering(event_id, room_id)
 
-            if existing_read_marker:
+            if existing_read_marker and not allow_backward:
                 try:
                     old_event_ordering = await self.store.get_event_ordering(
                         existing_read_marker["event_id"], room_id
diff --git a/synapse/rest/client/read_marker.py b/synapse/rest/client/read_marker.py
index e92903cd8..a21811d60 100644
--- a/synapse/rest/client/read_marker.py
+++ b/synapse/rest/client/read_marker.py
@@ -23,6 +23,7 @@
 from typing import TYPE_CHECKING, Tuple
 
 from synapse.api.constants import ReceiptTypes
+from synapse.api.errors import Codes, SynapseError
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
 from synapse.http.site import SynapseRequest
@@ -73,6 +74,19 @@ async def handle_read_marker(
         )
 
         unrecognized_types = set(body.keys()) - self._known_receipt_types
+
+        if self.config.experimental.msc4446_enabled:
+            allow_backward = body.get("com.beeper.allow_backward", False)
+            if not isinstance(allow_backward, bool):
+                raise SynapseError(
+                    400,
+                    "com.beeper.allow_backward must be a boolean.",
+                    Codes.INVALID_PARAM,
+                )
+            unrecognized_types -= {"com.beeper.allow_backward"}
+        else:
+            allow_backward = False
+
         if unrecognized_types:
             # It's fine if there are unrecognized receipt types, but let's log
             # it to help debug clients that have typoed the receipt type.
@@ -93,6 +107,7 @@ async def handle_read_marker(
                     room_id,
                     user_id=requester.user.to_string(),
                     event_id=event_id,
+                    allow_backward=allow_backward,
                     extra_content=body.get("com.beeper.fully_read.extra", None),
                 )
             else:
diff --git a/synapse/rest/client/receipts.py b/synapse/rest/client/receipts.py
index 11f7f3fb5..bb266d297 100644
--- a/synapse/rest/client/receipts.py
+++ b/synapse/rest/client/receipts.py
@@ -20,6 +20,7 @@
 #
 
 import logging
+from http import HTTPStatus
 from typing import TYPE_CHECKING, Tuple
 
 from synapse.api.constants import MAIN_TIMELINE, ReceiptTypes
@@ -50,6 +51,7 @@ def __init__(self, hs: "HomeServer"):
         self.read_marker_handler = hs.get_read_marker_handler()
         self.presence_handler = hs.get_presence_handler()
         self._main_store = hs.get_datastores().main
+        self._msc4446_enabled = hs.config.experimental.msc4446_enabled
 
         self._known_receipt_types = {
             ReceiptTypes.READ,
@@ -74,6 +76,25 @@ async def on_POST(
 
         body = parse_json_object_from_request(request, allow_empty_body=False)
 
+        if self._msc4446_enabled:
+            allow_backward = body.get("com.beeper.allow_backward", False)
+            if not isinstance(allow_backward, bool):
+                raise SynapseError(
+                    HTTPStatus.BAD_REQUEST,
+                    "com.beeper.allow_backward must be a boolean.",
+                    Codes.INVALID_PARAM,
+                )
+
+            if allow_backward and receipt_type != ReceiptTypes.FULLY_READ:
+                raise SynapseError(
+                    HTTPStatus.BAD_REQUEST,
+                    "com.beeper.allow_backward is only allowed to be true for "
+                    f"{ReceiptTypes.FULLY_READ}.",
+                    Codes.INVALID_PARAM,
+                )
+        else:
+            allow_backward = False
+
         # Pull the thread ID, if one exists.
         thread_id = None
         if "thread_id" in body:
@@ -109,6 +130,7 @@ async def on_POST(
                 room_id,
                 user_id=requester.user.to_string(),
                 event_id=event_id,
+                allow_backward=allow_backward,
                 extra_content=body,
             )
         else:
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index 20395430d..0673b0a7e 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -180,6 +180,8 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
                     "org.matrix.msc4155": self.config.experimental.msc4155_enabled,
                     # MSC4306: Support for thread subscriptions
                     "org.matrix.msc4306": self.config.experimental.msc4306_enabled,
+                    # MSC4446: Allow moving the fully read marker backwards.
+                    "com.beeper.msc4446": self.config.experimental.msc4446_enabled,
                     # MSC4169: Backwards-compatible redaction sending using `/send`
                     "com.beeper.msc4169": self.config.experimental.msc4169_enabled,
                 },