diff --git a/synapse/util/ratelimitutils.py b/synapse/util/ratelimitutils.py
index f262bf95a0fd..0b03d43cd2ad 100644
--- a/synapse/util/ratelimitutils.py
+++ b/synapse/util/ratelimitutils.py
@@ -29,6 +29,7 @@
     Set,
     Tuple,
 )
+from weakref import WeakSet
 
 from prometheus_client.core import Counter
 from typing_extensions import ContextManager
@@ -86,7 +87,9 @@
 )
 
 
-_rate_limiter_instances: Set["FederationRateLimiter"] = set()
+# This must be a `WeakSet`, otherwise we indirectly hold on to entire `HomeServer`s
+# during trial test runs and leak a lot of memory.
+_rate_limiter_instances: Set["FederationRateLimiter"] = WeakSet()
 # Protects the _rate_limiter_instances set from concurrent access
 _rate_limiter_instances_lock = threading.Lock()