diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index 55151ca549..090652961d 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -139,6 +139,9 @@ class FederationClient(FederationBase):
         self.server_name = hs.hostname
         self.signing_key = hs.signing_key
 
+        # Avoid a cyclic dependency between this and the room policy handler itself
+        self._get_room_policy_handler = hs.get_room_policy_handler
+
         # Cache mapping `event_id` to a tuple of the event itself and the `pull_origin`
         # (which server we pulled the event from)
         self._get_pdu_cache: ExpiringCache[str, tuple[EventBase, str]] = ExpiringCache(
@@ -1317,6 +1320,11 @@ class FederationClient(FederationBase):
 
         pdu = event_from_pdu_json(pdu_dict, room_version)
 
+        # Verify the policy server has allowed the event, raising a SynapseError if not.
+        # We use `raise_if_not_allowed` instead of asking for a signature because the
+        # event we received might already have a policy server signature.
+        await self._get_room_policy_handler().raise_if_not_allowed(pdu)
+
         # Check signatures are correct.
         try:
             pdu = await self._check_sigs_and_hash(room_version, pdu)
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index eff6d63789..1bd3b890c7 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -151,6 +151,7 @@ class FederationServer(FederationBase):
         self._state_storage_controller = hs.get_storage_controllers().state
 
         self.device_handler = hs.get_device_handler()
+        self._room_policy_handler = hs.get_room_policy_handler()
 
         # Ensure the following handlers are loaded since they register callbacks
         # with FederationHandlerRegistry.
@@ -741,6 +742,12 @@ class FederationServer(FederationBase):
                 pdu.event_id,
             )
             raise SynapseError(403, Codes.FORBIDDEN)
+
+        # Verify the policy server has allowed the event, raising a SynapseError if not.
+        # We use `raise_if_not_allowed` instead of asking for a signature because the
+        # event we received might already have a policy server signature.
+        await self._room_policy_handler.raise_if_not_allowed(pdu)
+
         try:
             pdu = await self._check_sigs_and_hash(room_version, pdu)
         except InvalidEventSignatureError as e:
@@ -1019,6 +1026,11 @@ class FederationServer(FederationBase):
                 )
             )
 
+        # Verify the policy server has allowed the event, raising a SynapseError if not.
+        # We use `raise_if_not_allowed` instead of asking for a signature because the
+        # event we received might already have a policy server signature.
+        await self._room_policy_handler.raise_if_not_allowed(event)
+
         try:
             event = await self._check_sigs_and_hash(room_version, event)
         except InvalidEventSignatureError as e:
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index b2e678e90e..ae6f4d79e5 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -107,6 +107,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self.account_data_handler = hs.get_account_data_handler()
         self.event_auth_handler = hs.get_event_auth_handler()
         self._worker_lock_handler = hs.get_worker_locks_handler()
+        self._room_policy_handler = hs.get_room_policy_handler()
 
         self._membership_types_to_include_profile_data_in = {
             Membership.JOIN,
@@ -496,6 +497,16 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
                     outlier=outlier,
                     delay_id=delay_id,
                 )
+
+                # Before we persist the event, run the event through the policy server
+                # to ensure the membership action can happen. This will raise a
+                # SynapseError if the policy server refused it, and otherwise append a
+                # signature to the event.
+                await self._room_policy_handler.ask_policy_server_to_sign_event(
+                    event,
+                    verify=True,
+                )
+
                 context = await unpersisted_context.persist(event)
                 prev_state_ids = await context.get_prev_state_ids(
                     StateFilter.from_types([(EventTypes.Member, user_id)])
diff --git a/synapse/handlers/room_policy.py b/synapse/handlers/room_policy.py
index 57163c4344..998a4a24cd 100644
--- a/synapse/handlers/room_policy.py
+++ b/synapse/handlers/room_policy.py
@@ -151,8 +151,40 @@ class RoomPolicyHandler:
         Returns:
             bool: True if the event is allowed in the room, False otherwise.
         """
+        try:
+            await self.raise_if_not_allowed(event)
+        except Exception as ex:
+            # We probably caught either a refusal to sign, an invalid signature, or
+            # some other transient or network error. These are all rejection cases.
+            logger.warning("Failed to get a signature from the policy server: %s", ex)
+            return False
+
+        return True
+
+    async def raise_if_not_allowed(self, event: EventBase) -> None:
+        """
+        Check if the given event is allowed in the room by the policy server, and raise
+        a SynapseError if not.
+
+        Note: This will *not* raise errors if the room's policy server is Synapse itself.
+        This is because Synapse can't be a policy server (currently).
+
+        If no policy server is configured in the room, this does not raise. Similarly, if
+        the policy server is invalid in any way (not joined, not a server, etc), this
+        does not raise.
+
+        If a valid and contactable policy server is configured in the room, this raises
+        a SynapseError if that server suggests the event is spammy, and returns nothing
+        otherwise.
+
+        Args:
+            event: The event to check. This should be a fully-formed PDU.
+
+        Raises:
+            SynapseError: If the event is spammy according to the policy server.
+        """
         if self._is_policy_server_state_event(event):
-            return True  # always allow policy server change events
+            return  # always allow policy server change events
 
         policy_server = await self._get_policy_server(event.room_id)
         if policy_server is None:
@@ -165,18 +197,13 @@ class RoomPolicyHandler:
             event, policy_server.server_name, policy_server.public_key
         )
         if valid:
-            return True  # valid signature == allow
+            return  # valid signature == allow
 
         # We couldn't save the HTTP hit, so do that hit.
-        try:
-            await self.ask_policy_server_to_sign_event(event, verify=True)
-        except Exception as ex:
-            # We probably caught either a refusal to sign, an invalid signature, or
-            # some other transient or network error. These are all rejection cases.
-            logger.warning("Failed to get a signature from the policy server: %s", ex)
-            return False
+        # This raises the SynapseError we mentioned in the docstring
+        await self.ask_policy_server_to_sign_event(event, verify=True)
 
-        return True  # passed all verifications and checks, so allow
+        return  # passed all verifications and checks, so allow
 
     async def _verify_policy_server_signature(
         self, event: EventBase, policy_server: str, public_key: str