diff --git a/hc/api/models.py b/hc/api/models.py
index 8bdd91ad..4561ce60 100644
--- a/hc/api/models.py
+++ b/hc/api/models.py
@@ -902,6 +902,14 @@ class TokenBucket(models.Model):
         # 6 messages for a single chat per minute:
         return TokenBucket.authorize(value, 6, 60)
 
+    @staticmethod
+    def authorize_signal(phone):
+        salted_encoded = (phone + settings.SECRET_KEY).encode()
+        value = "signal-%s" % hashlib.sha1(salted_encoded).hexdigest()
+
+        # 6 messages for a single recipient per minute:
+        return TokenBucket.authorize(value, 6, 60)
+
     @staticmethod
     def authorize_sudo_code(user):
         value = "sudo-%d" % user.id
diff --git a/hc/api/tests/test_notify_signal.py b/hc/api/tests/test_notify_signal.py
index 46f25f58..6b2d5eef 100644
--- a/hc/api/tests/test_notify_signal.py
+++ b/hc/api/tests/test_notify_signal.py
@@ -6,7 +6,7 @@ from unittest.mock import patch
 
 from django.utils.timezone import now
 from django.test.utils import override_settings
-from hc.api.models import Channel, Check, Notification
+from hc.api.models import Channel, Check, Notification, TokenBucket
 from hc.test import BaseTestCase
 
 
@@ -80,3 +80,14 @@ class NotifySignalTestCase(BaseTestCase):
         cmd = " ".join(args[0])
 
         self.assertIn("Foo & Bar", cmd)
+
+    @override_settings(SECRET_KEY="test-secret")
+    def test_it_obeys_rate_limit(self):
+        # "2862..." is sha1("+123456789test-secret")
+        obj = TokenBucket(value="signal-2862991ccaa15c8856e7ee0abaf3448fb3c292e0")
+        obj.tokens = 0
+        obj.save()
+
+        self.channel.notify(self.check)
+        n = Notification.objects.first()
+        self.assertEqual(n.error, "Rate limit exceeded")
diff --git a/hc/api/transports.py b/hc/api/transports.py
index 5349d06a..c5786df0 100644
--- a/hc/api/transports.py
+++ b/hc/api/transports.py
@@ -673,6 +673,11 @@ class Signal(Transport):
         if not settings.SIGNAL_CLI_USERNAME:
             return "Signal notifications are not enabled"
 
+        from hc.api.models import TokenBucket
+
+        if not TokenBucket.authorize_signal(self.channel.phone_number):
+            return "Rate limit exceeded"
+
         text = tmpl("signal_message.html", check=check, site_name=settings.SITE_NAME)
 
         args = settings.SIGNAL_CLI_CMD.split()