diff --git a/changelog.d/8364.bugfix b/changelog.d/8364.bugfix
new file mode 100644
index 000000000..7b82cbc38
--- /dev/null
+++ b/changelog.d/8364.bugfix
@@ -0,0 +1,2 @@
+Fix a bug where during device registration the length of the device name wasn't
+limited.
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 55a978743..4149520d6 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -20,6 +20,7 @@ from typing import Any, Dict, List, Optional
 from synapse.api import errors
 from synapse.api.constants import EventTypes
 from synapse.api.errors import (
+    Codes,
     FederationDeniedError,
     HttpResponseException,
     RequestSendFailed,
@@ -265,6 +266,24 @@ class DeviceHandler(DeviceWorkerHandler):
 
         hs.get_distributor().observe("user_left_room", self.user_left_room)
 
+    def _check_device_name_length(self, name: str):
+        """
+        Checks whether a device name is longer than the maximum allowed length.
+
+        Args:
+            name: The name of the device.
+
+        Raises:
+            SynapseError: if the device name is too long.
+        """
+        if name and len(name) > MAX_DEVICE_DISPLAY_NAME_LEN:
+            raise SynapseError(
+                400,
+                "Device display name is too long (max %i)"
+                % (MAX_DEVICE_DISPLAY_NAME_LEN,),
+                errcode=Codes.TOO_LARGE,
+            )
+
     async def check_device_registered(
         self, user_id, device_id, initial_device_display_name=None
     ):
@@ -282,6 +301,9 @@ class DeviceHandler(DeviceWorkerHandler):
         Returns:
             str: device id (generated if none was supplied)
         """
+
+        self._check_device_name_length(initial_device_display_name)
+
         if device_id is not None:
             new_device = await self.store.store_device(
                 user_id=user_id,
@@ -397,12 +419,8 @@ class DeviceHandler(DeviceWorkerHandler):
 
         # Reject a new displayname which is too long.
         new_display_name = content.get("display_name")
-        if new_display_name and len(new_display_name) > MAX_DEVICE_DISPLAY_NAME_LEN:
-            raise SynapseError(
-                400,
-                "Device display name is too long (max %i)"
-                % (MAX_DEVICE_DISPLAY_NAME_LEN,),
-            )
+
+        self._check_device_name_length(new_display_name)
 
         try:
             await self.store.update_device(
diff --git a/tests/handlers/test_device.py b/tests/handlers/test_device.py
index 6aa322bf3..969d44c78 100644
--- a/tests/handlers/test_device.py
+++ b/tests/handlers/test_device.py
@@ -35,6 +35,17 @@ class DeviceTestCase(unittest.HomeserverTestCase):
         # These tests assume that it starts 1000 seconds in.
         self.reactor.advance(1000)
 
+    def test_device_is_created_with_invalid_name(self):
+        self.get_failure(
+            self.handler.check_device_registered(
+                user_id="@boris:foo",
+                device_id="foo",
+                initial_device_display_name="a"
+                * (synapse.handlers.device.MAX_DEVICE_DISPLAY_NAME_LEN + 1),
+            ),
+            synapse.api.errors.SynapseError,
+        )
+
     def test_device_is_created_if_doesnt_exist(self):
         res = self.get_success(
             self.handler.check_device_registered(
diff --git a/tests/rest/admin/test_device.py b/tests/rest/admin/test_device.py
index faa7f381a..92c905888 100644
--- a/tests/rest/admin/test_device.py
+++ b/tests/rest/admin/test_device.py
@@ -221,7 +221,7 @@ class DeviceRestTestCase(unittest.HomeserverTestCase):
         self.render(request)
 
         self.assertEqual(400, channel.code, msg=channel.json_body)
-        self.assertEqual(Codes.UNKNOWN, channel.json_body["errcode"])
+        self.assertEqual(Codes.TOO_LARGE, channel.json_body["errcode"])
 
         # Ensure the display name was not updated.
         request, channel = self.make_request(