diff --git a/changelog.d/9655.feature b/changelog.d/9655.feature
new file mode 100644
index 0000000000..70cac230d8
--- /dev/null
+++ b/changelog.d/9655.feature
@@ -0,0 +1 @@
+Add [MSC3069](https://github.com/matrix-org/matrix-doc/pull/3069) support to `/account/whoami`.
\ No newline at end of file
diff --git a/synapse/rest/client/account.py b/synapse/rest/client/account.py
index 6a7608d60b..bacb828330 100644
--- a/synapse/rest/client/account.py
+++ b/synapse/rest/client/account.py
@@ -878,9 +878,13 @@ class WhoamiRestServlet(RestServlet):
         self.auth = hs.get_auth()
 
     async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
-        requester = await self.auth.get_user_by_req(request)
+        requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
-        response = {"user_id": requester.user.to_string()}
+        response = {
+            "user_id": requester.user.to_string(),
+            # MSC: https://github.com/matrix-org/matrix-doc/pull/3069
+            "org.matrix.msc3069.is_guest": bool(requester.is_guest),
+        }
 
         # Appservices and similar accounts do not have device IDs
         # that we can report on, so exclude them for compliance.
diff --git a/tests/rest/client/test_account.py b/tests/rest/client/test_account.py
index 9e9e953cf4..64b0b8458b 100644
--- a/tests/rest/client/test_account.py
+++ b/tests/rest/client/test_account.py
@@ -470,13 +470,45 @@ class WhoamiTestCase(unittest.HomeserverTestCase):
         register.register_servlets,
     ]
 
+    def default_config(self):
+        config = super().default_config()
+        config["allow_guest_access"] = True
+        return config
+
     def test_GET_whoami(self):
         device_id = "wouldgohere"
         user_id = self.register_user("kermit", "test")
         tok = self.login("kermit", "test", device_id=device_id)
 
-        whoami = self.whoami(tok)
-        self.assertEqual(whoami, {"user_id": user_id, "device_id": device_id})
+        whoami = self._whoami(tok)
+        self.assertEqual(
+            whoami,
+            {
+                "user_id": user_id,
+                "device_id": device_id,
+                # Unstable until MSC3069 enters spec
+                "org.matrix.msc3069.is_guest": False,
+            },
+        )
+
+    def test_GET_whoami_guests(self):
+        channel = self.make_request(
+            b"POST", b"/_matrix/client/r0/register?kind=guest", b"{}"
+        )
+        tok = channel.json_body["access_token"]
+        user_id = channel.json_body["user_id"]
+        device_id = channel.json_body["device_id"]
+
+        whoami = self._whoami(tok)
+        self.assertEqual(
+            whoami,
+            {
+                "user_id": user_id,
+                "device_id": device_id,
+                # Unstable until MSC3069 enters spec
+                "org.matrix.msc3069.is_guest": True,
+            },
+        )
 
     def test_GET_whoami_appservices(self):
         user_id = "@as:test"
@@ -491,11 +523,18 @@ class WhoamiTestCase(unittest.HomeserverTestCase):
         )
         self.hs.get_datastore().services_cache.append(appservice)
 
-        whoami = self.whoami(as_token)
-        self.assertEqual(whoami, {"user_id": user_id})
+        whoami = self._whoami(as_token)
+        self.assertEqual(
+            whoami,
+            {
+                "user_id": user_id,
+                # Unstable until MSC3069 enters spec
+                "org.matrix.msc3069.is_guest": False,
+            },
+        )
         self.assertFalse(hasattr(whoami, "device_id"))
 
-    def whoami(self, tok):
+    def _whoami(self, tok):
         channel = self.make_request("GET", "account/whoami", {}, access_token=tok)
         self.assertEqual(channel.code, 200)
         return channel.json_body