diff --git a/CHANGELOG.md b/CHANGELOG.md
index 24ff786e..6f41b7c4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ All notable changes to this project will be documented in this file.
 ### Bug Fixes
 - Removing Pager Team integration, project appears to be discontinued
 - Sending a test notification updates Channel.last_error (#391)
+- Handle HTTP 429 responses from Matrix server when joining a Matrix room
 
 ## v1.15.0 - 2020-06-04
 
diff --git a/hc/front/forms.py b/hc/front/forms.py
index bb35decb..3aaabc96 100644
--- a/hc/front/forms.py
+++ b/hc/front/forms.py
@@ -207,7 +207,14 @@ class AddMatrixForm(forms.Form):
         url = settings.MATRIX_HOMESERVER
         url += "/_matrix/client/r0/join/%s?" % quote(v)
         url += urlencode({"access_token": settings.MATRIX_ACCESS_TOKEN})
-        doc = requests.post(url, {}).json()
+        r = requests.post(url, {})
+        if r.status_code == 429:
+            raise forms.ValidationError(
+                "Matrix server returned status code 429 (Too Many Requests), "
+                "please try again later."
+            )
+
+        doc = r.json()
         if "error" in doc:
             raise forms.ValidationError("Response from Matrix: %s" % doc["error"])
 
diff --git a/hc/front/tests/test_add_matrix.py b/hc/front/tests/test_add_matrix.py
index fe729c94..d7b32e41 100644
--- a/hc/front/tests/test_add_matrix.py
+++ b/hc/front/tests/test_add_matrix.py
@@ -1,3 +1,4 @@
+from json import JSONDecodeError
 from unittest.mock import patch
 
 from django.test.utils import override_settings
@@ -37,3 +38,14 @@ class AddMatrixTestCase(BaseTestCase):
         self.client.login(username="alice@example.org", password="password")
         r = self.client.get(self.url)
         self.assertEqual(r.status_code, 404)
+
+    @patch("hc.front.forms.requests.post")
+    def test_it_handles_429(self, mock_post):
+        mock_post.return_value.status_code = 429
+
+        form = {"alias": "!foo:example.org"}
+        self.client.login(username="alice@example.org", password="password")
+        r = self.client.post(self.url, form)
+
+        self.assertContains(r, "Matrix server returned status code 429")
+        self.assertFalse(Channel.objects.exists())