diff --git a/synapse/http/federation/matrix_federation_agent.py b/synapse/http/federation/matrix_federation_agent.py
index a0d513983..79e488c94 100644
--- a/synapse/http/federation/matrix_federation_agent.py
+++ b/synapse/http/federation/matrix_federation_agent.py
@@ -47,6 +47,9 @@ WELL_KNOWN_INVALID_CACHE_PERIOD = 1 * 3600
 # cap for .well-known cache period
 WELL_KNOWN_MAX_CACHE_PERIOD = 48 * 3600
 
+# lower bound for .well-known cache period
+WELL_KNOWN_MIN_CACHE_PERIOD = 5 * 60
+
 logger = logging.getLogger(__name__)
 well_known_cache = TTLCache("well-known")
 
@@ -356,6 +359,7 @@ class MatrixFederationAgent(object):
             cache_period += random.uniform(0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER)
         else:
             cache_period = min(cache_period, WELL_KNOWN_MAX_CACHE_PERIOD)
+            cache_period = max(cache_period, WELL_KNOWN_MIN_CACHE_PERIOD)
 
         return (result, cache_period)
 
diff --git a/tests/http/federation/test_matrix_federation_agent.py b/tests/http/federation/test_matrix_federation_agent.py
index 4255add09..5e709c0c1 100644
--- a/tests/http/federation/test_matrix_federation_agent.py
+++ b/tests/http/federation/test_matrix_federation_agent.py
@@ -953,7 +953,7 @@ class MatrixFederationAgentTests(TestCase):
         well_known_server = self._handle_well_known_connection(
             client_factory,
             expected_sni=b"testserv",
-            response_headers={b"Cache-Control": b"max-age=10"},
+            response_headers={b"Cache-Control": b"max-age=1000"},
             content=b'{ "m.server": "target-server" }',
         )
 
@@ -969,7 +969,7 @@ class MatrixFederationAgentTests(TestCase):
         self.assertEqual(r, b"target-server")
 
         # expire the cache
-        self.reactor.pump((10.0,))
+        self.reactor.pump((1000.0,))
 
         # now it should connect again
         fetch_d = self.do_get_well_known(b"testserv")