diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index 429e79c47..0355d020e 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -190,6 +190,22 @@ event_processing_last_ts = Gauge("synapse_event_processing_last_ts", "", ["name"
 # finished being processed.
 event_processing_lag = Gauge("synapse_event_processing_lag", "", ["name"])
 
+last_ticked = time.time()
+
+
+class ReactorLastSeenMetric(object):
+
+    def collect(self):
+        cm = GaugeMetricFamily(
+            "python_twisted_reactor_last_seen",
+            "Seconds since the Twisted reactor was last seen",
+        )
+        cm.add_metric([], time.time() - last_ticked)
+        yield cm
+
+
+REGISTRY.register(ReactorLastSeenMetric())
+
 
 def runUntilCurrentTimer(func):
 
@@ -222,6 +238,11 @@ def runUntilCurrentTimer(func):
         tick_time.observe(end - start)
         pending_calls_metric.observe(num_pending)
 
+        # Update the time we last ticked, for the metric to test whether
+        # Synapse's reactor has frozen
+        global last_ticked
+        last_ticked = end
+
         if running_on_pypy:
             return ret