diff --git a/synapse/events/validator.py b/synapse/events/validator.py
index 463fff0c8ab7..66341c3372a6 100644
--- a/synapse/events/validator.py
+++ b/synapse/events/validator.py
@@ -12,9 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import collections.abc
-from typing import Iterable, Type, Union, cast
+from typing import Iterable, List, Type, Union, cast
 
 import jsonschema
+from pydantic import StrictBool, StrictStr, Field
 
 from synapse.api.constants import (
     MAX_ALIAS_LENGTH,
@@ -33,6 +34,8 @@
     validate_canonicaljson,
 )
 from synapse.federation.federation_server import server_matches_acl_event
+from synapse.http.servlet import validate_json_object
+from synapse.rest.models import RequestBodyModel
 from synapse.types import EventID, JsonDict, RoomID, UserID
 
 
@@ -135,13 +138,9 @@ def validate_new(self, event: EventBase, config: HomeServerConfig) -> None:
             EventContentFields.MSC3952_MENTIONS in event.content
             and config.experimental.msc3952_intentional_mentions
         ):
-            mentions = event.content[EventContentFields.MSC3952_MENTIONS]
-            try:
-                jsonschema.validate(
-                    instance=mentions, schema=MENTIONS_SCHEMA, cls=MENTIONS_VALIDATOR
-                )
-            except jsonschema.ValidationError as e:
-                raise SynapseError(400, e.message)
+            validate_json_object(
+                event.content[EventContentFields.MSC3952_MENTIONS], Mentions
+            )
 
     def _validate_retention(self, event: EventBase) -> None:
         """Checks that an event that defines the retention policy for a room respects the
@@ -270,20 +269,10 @@ def _ensure_state_event(self, event: Union[EventBase, EventBuilder]) -> None:
     },
 }
 
-MENTIONS_SCHEMA = {
-    "type": "object",
-    "properties": {
-        "user_ids": {
-            "type": "array",
-            "items": {
-                "type": "string",
-            },
-        },
-        "room": {
-            "type": "boolean",
-        },
-    },
-}
+
+class Mentions(RequestBodyModel):
+    user_ids: List[StrictStr] = Field(default_factory=list)
+    room: StrictBool = False
 
 
 # This could return something newer than Draft 7, but that's the current "latest"
@@ -302,4 +291,3 @@ def _create_validator(schema: JsonDict) -> Type[jsonschema.Draft7Validator]:
 
 
 POWER_LEVELS_VALIDATOR = _create_validator(POWER_LEVELS_SCHEMA)
-MENTIONS_VALIDATOR = _create_validator(MENTIONS_SCHEMA)
diff --git a/synapse/http/servlet.py b/synapse/http/servlet.py
index 0070bd2940ea..fc6279362881 100644
--- a/synapse/http/servlet.py
+++ b/synapse/http/servlet.py
@@ -778,17 +778,13 @@ def parse_json_object_from_request(
 Model = TypeVar("Model", bound=BaseModel)
 
 
-def parse_and_validate_json_object_from_request(
-    request: Request, model_type: Type[Model]
-) -> Model:
-    """Parse a JSON object from the body of a twisted HTTP request, then deserialise and
-    validate using the given pydantic model.
+def validate_json_object(content: JsonDict, model_type: Type[Model]) -> Model:
+    """Validate a deserialized JSON object using the given pydantic model.
 
     Raises:
         SynapseError if the request body couldn't be decoded as JSON or
             if it wasn't a JSON object.
     """
-    content = parse_json_object_from_request(request, allow_empty_body=False)
     try:
         instance = model_type.parse_obj(content)
     except ValidationError as e:
@@ -811,6 +807,20 @@ def parse_and_validate_json_object_from_request(
     return instance
 
 
+def parse_and_validate_json_object_from_request(
+    request: Request, model_type: Type[Model]
+) -> Model:
+    """Parse a JSON object from the body of a twisted HTTP request, then deserialise and
+    validate using the given pydantic model.
+
+    Raises:
+        SynapseError if the request body couldn't be decoded as JSON or
+            if it wasn't a JSON object.
+    """
+    content = parse_json_object_from_request(request, allow_empty_body=False)
+    return validate_json_object(content, model_type)
+
+
 def assert_params_in_dict(body: JsonDict, required: Iterable[str]) -> None:
     absent = []
     for k in required: