diff --git a/src/matrix/e2ee/RoomEncryption.js b/src/matrix/e2ee/RoomEncryption.js
index 7d540784..190852d3 100644
--- a/src/matrix/e2ee/RoomEncryption.js
+++ b/src/matrix/e2ee/RoomEncryption.js
@@ -45,6 +45,9 @@ export class RoomEncryption {
     }
 
     async writeMemberChanges(memberChanges, txn) {
+        if (memberChanges.some(m => m.hasLeft)) {
+            this._megolmEncryption.discardOutboundSession(this._room.id, txn);
+        }
         return await this._deviceTracker.writeMemberChanges(this._room, memberChanges, txn);
     }
 
diff --git a/src/matrix/e2ee/megolm/Encryption.js b/src/matrix/e2ee/megolm/Encryption.js
index d3c613f6..9b7df4eb 100644
--- a/src/matrix/e2ee/megolm/Encryption.js
+++ b/src/matrix/e2ee/megolm/Encryption.js
@@ -26,6 +26,10 @@ export class Encryption {
         this._ownDeviceId = ownDeviceId;
     }
 
+    discardOutboundSession(roomId, txn) {
+        txn.outboundGroupSessions.remove(roomId);
+    }
+
     /**
      * Encrypts a message with megolm
      * @param  {string} roomId           
diff --git a/src/matrix/room/members/RoomMember.js b/src/matrix/room/members/RoomMember.js
index 954de0a4..6b13c721 100644
--- a/src/matrix/room/members/RoomMember.js
+++ b/src/matrix/room/members/RoomMember.js
@@ -134,4 +134,8 @@ export class MemberChange {
     get membership() {
         return this._memberEvent.content?.membership;
     }
+
+    get hasLeft() {
+        return this.previousMembership === "join" && this.membership !== "join";
+    }
 }
diff --git a/src/matrix/storage/idb/stores/OutboundGroupSessionStore.js b/src/matrix/storage/idb/stores/OutboundGroupSessionStore.js
index ef9224be..9710765f 100644
--- a/src/matrix/storage/idb/stores/OutboundGroupSessionStore.js
+++ b/src/matrix/storage/idb/stores/OutboundGroupSessionStore.js
@@ -19,6 +19,10 @@ export class OutboundGroupSessionStore {
         this._store = store;
     }
 
+    remove(roomId) {
+        this._store.delete(roomId);
+    }
+
     get(roomId) {
         return this._store.get(roomId);
     }