diff --git a/changelog.d/18660.bugfix b/changelog.d/18660.bugfix
new file mode 100644
index 000000000..fd65b43a1
--- /dev/null
+++ b/changelog.d/18660.bugfix
@@ -0,0 +1 @@
+Don't allow creation of tags with names longer than 255 bytes, as per the spec.
\ No newline at end of file
diff --git a/synapse/rest/client/tags.py b/synapse/rest/client/tags.py
index b6648f349..fb59efb11 100644
--- a/synapse/rest/client/tags.py
+++ b/synapse/rest/client/tags.py
@@ -20,9 +20,10 @@
 #
 
 import logging
+from http import HTTPStatus
 from typing import TYPE_CHECKING, Tuple
 
-from synapse.api.errors import AuthError
+from synapse.api.errors import AuthError, 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
@@ -35,6 +36,8 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
+MAX_TAG_LENGTH = 255
+
 
 class TagListServlet(RestServlet):
     """
@@ -86,6 +89,16 @@ class TagServlet(RestServlet):
         requester = await self.auth.get_user_by_req(request)
         if user_id != requester.user.to_string():
             raise AuthError(403, "Cannot add tags for other users.")
+
+        # check if the tag exceeds the length allowed by the matrix-specification
+        # as defined in: https://spec.matrix.org/v1.15/client-server-api/#events-14
+        if len(tag.encode("utf-8")) > MAX_TAG_LENGTH:
+            raise SynapseError(
+                HTTPStatus.BAD_REQUEST,
+                "tag parameter's length is over 255 bytes",
+                errcode=Codes.INVALID_PARAM,
+            )
+
         # Check if the user has any membership in the room and raise error if not.
         # Although it's not harmful for users to tag random rooms, it's just superfluous
         # data we don't need to track or allow.
diff --git a/tests/rest/client/test_tags.py b/tests/rest/client/test_tags.py
index 5d596409e..aee2f6aff 100644
--- a/tests/rest/client/test_tags.py
+++ b/tests/rest/client/test_tags.py
@@ -15,6 +15,7 @@
 """Tests REST events for /tags paths."""
 
 from http import HTTPStatus
+from urllib import parse as urlparse
 
 import synapse.rest.admin
 from synapse.rest.client import login, room, tags
@@ -93,3 +94,68 @@ class RoomTaggingTestCase(unittest.HomeserverTestCase):
         )
         # Check that the request failed with the correct error
         self.assertEqual(channel.code, HTTPStatus.FORBIDDEN, channel.result)
+
+    def test_put_tag_fails_if_tag_is_too_long(self) -> None:
+        """
+        Test that a user cannot add a tag to a room that is longer than the 255 bytes
+        allowed by the matrix specification.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        room_id = self.helper.create_room_as(user1_id, tok=user1_tok)
+        # create a string which is larger than 255 bytes
+        tag = "X" * 300
+
+        # Make the request
+        channel = self.make_request(
+            "PUT",
+            f"/user/{user1_id}/rooms/{room_id}/tags/{tag}",
+            content={"order": 0.5},
+            access_token=user1_tok,
+        )
+        # Check that the request failed
+        self.assertEqual(channel.code, HTTPStatus.BAD_REQUEST, channel.result)
+
+    def test_put_tag_fails_if_tag_is_too_long_with_graphemes(self) -> None:
+        """
+        Test that a user cannot add a tag to a room that contains graphemes which are in total
+        longer than the 255 bytes allowed by the matrix specification.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        room_id = self.helper.create_room_as(user1_id, tok=user1_tok)
+        # create a string which is larger than 255 bytes (275)
+        tag = "👩‍🚒" * 25
+
+        # Make the request
+        channel = self.make_request(
+            "PUT",
+            f"/user/{user1_id}/rooms/{room_id}/tags/"
+            + urlparse.quote(tag.encode("utf-8")),
+            content={"order": 0.5},
+            access_token=user1_tok,
+        )
+        # Check that the request failed
+        self.assertEqual(channel.code, HTTPStatus.BAD_REQUEST, channel.result)
+
+    def test_put_tag_succeeds_with_graphemes(self) -> None:
+        """
+        Test that a user can add a tag to a room that contains graphemes which are in total
+        less than the 255 bytes allowed by the matrix specification.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        room_id = self.helper.create_room_as(user1_id, tok=user1_tok)
+        # create a string of acceptable length (220 bytes)
+        tag = "👩‍🚒" * 20
+
+        # Make the request
+        channel = self.make_request(
+            "PUT",
+            f"/user/{user1_id}/rooms/{room_id}/tags/"
+            + urlparse.quote(tag.encode("utf-8")),
+            content={"order": 0.5},
+            access_token=user1_tok,
+        )
+        # Check that the request succeeded
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.result)