diff --git a/include/olm/inbound_group_session.h b/include/olm/inbound_group_session.h
index f8a0bc3..3e5a8b2 100644
--- a/include/olm/inbound_group_session.h
+++ b/include/olm/inbound_group_session.h
@@ -165,6 +165,13 @@ size_t olm_inbound_group_session_id(
     uint8_t * id, size_t id_length
 );
 
+/**
+ * Get the first message index we know how to decrypt.
+ */
+uint32_t olm_inbound_group_session_first_known_index(
+    const OlmInboundGroupSession *session
+);
+
 
 #ifdef __cplusplus
 } // extern "C"
diff --git a/javascript/olm_inbound_group_session.js b/javascript/olm_inbound_group_session.js
index 1b7fcfe..5fd56b3 100644
--- a/javascript/olm_inbound_group_session.js
+++ b/javascript/olm_inbound_group_session.js
@@ -105,4 +105,10 @@ InboundGroupSession.prototype['session_id'] = restore_stack(function() {
     return Pointer_stringify(session_id);
 });
 
+InboundGroupSession.prototype['first_known_index'] = restore_stack(function() {
+    return inbound_group_session_method(
+        Module['_olm_inbound_group_session_first_known_index']
+    )(this.ptr);
+});
+
 olm_exports['InboundGroupSession'] = InboundGroupSession;
diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c
index a54e55f..6f3c7e0 100644
--- a/src/inbound_group_session.c
+++ b/src/inbound_group_session.c
@@ -391,3 +391,9 @@ size_t olm_inbound_group_session_id(
         session->signing_key.public_key, GROUP_SESSION_ID_LENGTH, id
     );
 }
+
+uint32_t olm_inbound_group_session_first_known_index(
+    const OlmInboundGroupSession *session
+) {
+    return session->initial_ratchet.counter;
+}