diff --git a/specs/altair/light-client/sync-protocol.md b/specs/altair/light-client/sync-protocol.md
index f85c55258d..6d212300f7 100644
--- a/specs/altair/light-client/sync-protocol.md
+++ b/specs/altair/light-client/sync-protocol.md
@@ -272,6 +272,8 @@ def is_better_update(new_update: LightClientUpdate, old_update: LightClientUpdat
     # Tiebreaker 2: Prefer older data (fewer changes to best)
     if new_update.attested_header.beacon.slot != old_update.attested_header.beacon.slot:
         return new_update.attested_header.beacon.slot < old_update.attested_header.beacon.slot
+
+    # Tiebreaker 3: Prefer updates with earlier signature slots
     return new_update.signature_slot < old_update.signature_slot
 ```
 
diff --git a/tests/core/pyspec/eth2spec/test/altair/light_client/test_update_ranking.py b/tests/core/pyspec/eth2spec/test/altair/light_client/test_update_ranking.py
index 5bca370ab5..4f8575bd50 100644
--- a/tests/core/pyspec/eth2spec/test/altair/light_client/test_update_ranking.py
+++ b/tests/core/pyspec/eth2spec/test/altair/light_client/test_update_ranking.py
@@ -16,7 +16,7 @@
 )
 
 
-def create_test_update(spec, test, with_next, with_finality, participation_rate):
+def create_test_update(spec, test, with_next, with_finality, participation_rate, signature_slot=None):
     attested_state, attested_block, finalized_block = test
     return create_update(
         spec,
@@ -26,6 +26,7 @@ def create_test_update(spec, test, with_next, with_finality, participation_rate)
         with_next,
         with_finality,
         participation_rate,
+        signature_slot=signature_slot,
     )
 
 
@@ -132,6 +133,15 @@ def test_update_ranking(spec, state):
         create_test_update(spec, att, with_next=0, with_finality=0, participation_rate=0.2),
         create_test_update(spec, fin, with_next=0, with_finality=0, participation_rate=0.2),
         create_test_update(spec, lat, with_next=0, with_finality=0, participation_rate=0.2),
+        # Test signature_slot tiebreaker: identical update but with later signature_slot
+        create_test_update(
+            spec,
+            lat,
+            with_next=0,
+            with_finality=0,
+            participation_rate=0.2,
+            signature_slot=lat_attested_state.slot + 2,
+        ),
     ]
     yield "updates", updates
 
diff --git a/tests/core/pyspec/eth2spec/test/helpers/light_client.py b/tests/core/pyspec/eth2spec/test/helpers/light_client.py
index c9334788ae..b3c1a914be 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/light_client.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/light_client.py
@@ -86,7 +86,8 @@ def create_update(spec,
                   finalized_block,
                   with_next,
                   with_finality,
-                  participation_rate):
+                  participation_rate,
+                  signature_slot=None):
     num_participants = floor(spec.SYNC_COMMITTEE_SIZE * participation_rate)
 
     update = spec.LightClientUpdate()
@@ -104,7 +105,7 @@ def create_update(spec,
             attested_state, latest_finalized_root_gindex(spec))
 
     update.sync_aggregate, update.signature_slot = get_sync_aggregate(
-        spec, attested_state, num_participants)
+        spec, attested_state, num_participants, signature_slot=signature_slot)
 
     return update