diff --git a/changelog.d/11430.misc b/changelog.d/11430.misc
new file mode 100644
index 0000000000..28f06f4c4e
--- /dev/null
+++ b/changelog.d/11430.misc
@@ -0,0 +1 @@
+Update [MSC2918 refresh token](https://github.com/matrix-org/matrix-doc/blob/main/proposals/2918-refreshtokens.md#msc2918-refresh-tokens) support to confirm with the latest revision: accept the `refresh_tokens` parameter in the request body rather than in the URL parameters.
\ No newline at end of file
diff --git a/synapse/rest/client/login.py b/synapse/rest/client/login.py
index c982e54156..09f378f919 100644
--- a/synapse/rest/client/login.py
+++ b/synapse/rest/client/login.py
@@ -38,7 +38,6 @@ from synapse.http.server import HttpServer, finish_request
 from synapse.http.servlet import (
     RestServlet,
     assert_params_in_dict,
-    parse_boolean,
     parse_bytes_from_args,
     parse_json_object_from_request,
     parse_string,
@@ -165,11 +164,14 @@ class LoginRestServlet(RestServlet):
         login_submission = parse_json_object_from_request(request)
 
         if self._msc2918_enabled:
-            # Check if this login should also issue a refresh token, as per
-            # MSC2918
-            should_issue_refresh_token = parse_boolean(
-                request, name=LoginRestServlet.REFRESH_TOKEN_PARAM, default=False
+            # Check if this login should also issue a refresh token, as per MSC2918
+            should_issue_refresh_token = login_submission.get(
+                "org.matrix.msc2918.refresh_token", False
             )
+            if not isinstance(should_issue_refresh_token, bool):
+                raise SynapseError(
+                    400, "`org.matrix.msc2918.refresh_token` should be true or false."
+                )
         else:
             should_issue_refresh_token = False
 
diff --git a/synapse/rest/client/register.py b/synapse/rest/client/register.py
index d2b11e39d9..11fd6cd24d 100644
--- a/synapse/rest/client/register.py
+++ b/synapse/rest/client/register.py
@@ -41,7 +41,6 @@ from synapse.http.server import HttpServer, finish_request, respond_with_html
 from synapse.http.servlet import (
     RestServlet,
     assert_params_in_dict,
-    parse_boolean,
     parse_json_object_from_request,
     parse_string,
 )
@@ -449,9 +448,13 @@ class RegisterRestServlet(RestServlet):
         if self._msc2918_enabled:
             # Check if this registration should also issue a refresh token, as
             # per MSC2918
-            should_issue_refresh_token = parse_boolean(
-                request, name="org.matrix.msc2918.refresh_token", default=False
+            should_issue_refresh_token = body.get(
+                "org.matrix.msc2918.refresh_token", False
             )
+            if not isinstance(should_issue_refresh_token, bool):
+                raise SynapseError(
+                    400, "`org.matrix.msc2918.refresh_token` should be true or false."
+                )
         else:
             should_issue_refresh_token = False
 
diff --git a/tests/rest/client/test_auth.py b/tests/rest/client/test_auth.py
index 8045b7f76a..d8a94f4c12 100644
--- a/tests/rest/client/test_auth.py
+++ b/tests/rest/client/test_auth.py
@@ -529,7 +529,11 @@ class RefreshAuthTests(unittest.HomeserverTestCase):
         A login response should include a refresh_token only if asked.
         """
         # Test login
-        body = {"type": "m.login.password", "user": "test", "password": self.user_pass}
+        body = {
+            "type": "m.login.password",
+            "user": "test",
+            "password": self.user_pass,
+        }
 
         login_without_refresh = self.make_request(
             "POST", "/_matrix/client/r0/login", body
@@ -539,8 +543,8 @@ class RefreshAuthTests(unittest.HomeserverTestCase):
 
         login_with_refresh = self.make_request(
             "POST",
-            "/_matrix/client/r0/login?org.matrix.msc2918.refresh_token=true",
-            body,
+            "/_matrix/client/r0/login",
+            {"org.matrix.msc2918.refresh_token": True, **body},
         )
         self.assertEqual(login_with_refresh.code, 200, login_with_refresh.result)
         self.assertIn("refresh_token", login_with_refresh.json_body)
@@ -566,11 +570,12 @@ class RefreshAuthTests(unittest.HomeserverTestCase):
 
         register_with_refresh = self.make_request(
             "POST",
-            "/_matrix/client/r0/register?org.matrix.msc2918.refresh_token=true",
+            "/_matrix/client/r0/register",
             {
                 "username": "test3",
                 "password": self.user_pass,
                 "auth": {"type": LoginType.DUMMY},
+                "org.matrix.msc2918.refresh_token": True,
             },
         )
         self.assertEqual(register_with_refresh.code, 200, register_with_refresh.result)
@@ -581,10 +586,15 @@ class RefreshAuthTests(unittest.HomeserverTestCase):
         """
         A refresh token can be used to issue a new access token.
         """
-        body = {"type": "m.login.password", "user": "test", "password": self.user_pass}
+        body = {
+            "type": "m.login.password",
+            "user": "test",
+            "password": self.user_pass,
+            "org.matrix.msc2918.refresh_token": True,
+        }
         login_response = self.make_request(
             "POST",
-            "/_matrix/client/r0/login?org.matrix.msc2918.refresh_token=true",
+            "/_matrix/client/r0/login",
             body,
         )
         self.assertEqual(login_response.code, 200, login_response.result)
@@ -614,10 +624,15 @@ class RefreshAuthTests(unittest.HomeserverTestCase):
         """
         The access token should have some time as specified in the config.
         """
-        body = {"type": "m.login.password", "user": "test", "password": self.user_pass}
+        body = {
+            "type": "m.login.password",
+            "user": "test",
+            "password": self.user_pass,
+            "org.matrix.msc2918.refresh_token": True,
+        }
         login_response = self.make_request(
             "POST",
-            "/_matrix/client/r0/login?org.matrix.msc2918.refresh_token=true",
+            "/_matrix/client/r0/login",
             body,
         )
         self.assertEqual(login_response.code, 200, login_response.result)
@@ -666,10 +681,15 @@ class RefreshAuthTests(unittest.HomeserverTestCase):
         refresh the session.
         """
 
-        body = {"type": "m.login.password", "user": "test", "password": self.user_pass}
+        body = {
+            "type": "m.login.password",
+            "user": "test",
+            "password": self.user_pass,
+            "org.matrix.msc2918.refresh_token": True,
+        }
         login_response = self.make_request(
             "POST",
-            "/_matrix/client/r0/login?org.matrix.msc2918.refresh_token=true",
+            "/_matrix/client/r0/login",
             body,
         )
         self.assertEqual(login_response.code, HTTPStatus.OK, login_response.result)
@@ -711,10 +731,15 @@ class RefreshAuthTests(unittest.HomeserverTestCase):
         The session can be configured to have an ultimate, limited lifetime.
         """
 
-        body = {"type": "m.login.password", "user": "test", "password": self.user_pass}
+        body = {
+            "type": "m.login.password",
+            "user": "test",
+            "password": self.user_pass,
+            "org.matrix.msc2918.refresh_token": True,
+        }
         login_response = self.make_request(
             "POST",
-            "/_matrix/client/r0/login?org.matrix.msc2918.refresh_token=true",
+            "/_matrix/client/r0/login",
             body,
         )
         self.assertEqual(login_response.code, 200, login_response.result)
@@ -763,10 +788,15 @@ class RefreshAuthTests(unittest.HomeserverTestCase):
                    |-> fourth_refresh (fails)
         """
 
-        body = {"type": "m.login.password", "user": "test", "password": self.user_pass}
+        body = {
+            "type": "m.login.password",
+            "user": "test",
+            "password": self.user_pass,
+            "org.matrix.msc2918.refresh_token": True,
+        }
         login_response = self.make_request(
             "POST",
-            "/_matrix/client/r0/login?org.matrix.msc2918.refresh_token=true",
+            "/_matrix/client/r0/login",
             body,
         )
         self.assertEqual(login_response.code, 200, login_response.result)