diff --git a/changelog.d/5658.bugfix b/changelog.d/5658.bugfix
new file mode 100644
index 0000000000..f6ae906a9a
--- /dev/null
+++ b/changelog.d/5658.bugfix
@@ -0,0 +1 @@
+Fix 3PID bind requests being sent to identity servers as `application/x-form-www-urlencoded` data, which is deprecated.
diff --git a/synapse/handlers/identity.py b/synapse/handlers/identity.py
index 0b40541570..8696682c76 100644
--- a/synapse/handlers/identity.py
+++ b/synapse/handlers/identity.py
@@ -136,7 +136,7 @@ class IdentityHandler(BaseHandler):
             id_server_host = id_server
 
         try:
-            data = yield self.http_client.post_urlencoded_get_json(
+            data = yield self.http_client.post_json_get_json(
                 "https://%s%s" % (id_server_host, "/_matrix/identity/api/v1/3pid/bind"),
                 {"sid": creds["sid"], "client_secret": client_secret, "mxid": mxid},
             )
diff --git a/tests/handlers/test_identity.py b/tests/handlers/test_identity.py
index b8ecdc5c9b..32c31b2f66 100644
--- a/tests/handlers/test_identity.py
+++ b/tests/handlers/test_identity.py
@@ -43,8 +43,8 @@ class ThreepidISRewrittenURLTestCase(unittest.HomeserverTestCase):
             self.is_server_name: self.rewritten_is_url
         }
 
-        mock_http_client = Mock(spec=["post_urlencoded_get_json"])
-        mock_http_client.post_urlencoded_get_json.return_value = defer.succeed(
+        mock_http_client = Mock(spec=["post_json_get_json"])
+        mock_http_client.post_json_get_json.return_value = defer.succeed(
             {"address": self.address, "medium": "email"}
         )
 
@@ -65,7 +65,7 @@ class ThreepidISRewrittenURLTestCase(unittest.HomeserverTestCase):
         * the original, non-rewritten, server name is stored in the database
         """
         handler = self.hs.get_handlers().identity_handler
-        post_urlenc_get_json = self.hs.get_simple_http_client().post_urlencoded_get_json
+        post_json_get_json = self.hs.get_simple_http_client().post_json_get_json
         store = self.hs.get_datastore()
 
         creds = {"sid": "123", "client_secret": "some_secret"}
@@ -84,7 +84,7 @@ class ThreepidISRewrittenURLTestCase(unittest.HomeserverTestCase):
         self.assertEqual(data.get("address"), self.address)
 
         # Check that the request was done against the rewritten server name.
-        post_urlenc_get_json.assert_called_once_with(
+        post_json_get_json.assert_called_once_with(
             "https://%s/_matrix/identity/api/v1/3pid/bind" % self.rewritten_is_url,
             {
                 "sid": creds["sid"],