diff --git a/lib/shared/dm-ops/leave-thread-spec.js b/lib/shared/dm-ops/leave-thread-spec.js
--- a/lib/shared/dm-ops/leave-thread-spec.js
+++ b/lib/shared/dm-ops/leave-thread-spec.js
@@ -9,8 +9,12 @@
 } from './dm-op-spec.js';
 import type { DMLeaveThreadOperation } from '../../types/dm-ops.js';
 import { messageTypes } from '../../types/message-types-enum.js';
+import type { ThickRawThreadInfo } from '../../types/minimally-encoded-thread-permissions-types.js';
 import { threadTypes } from '../../types/thread-types-enum.js';
+import type { RawThreadInfos } from '../../types/thread-types.js';
 import { updateTypes } from '../../types/update-types-enum.js';
+import type { ClientUpdateInfo } from '../../types/update-types.js';
+import { values } from '../../utils/objects.js';
 import { rawMessageInfoFromMessageData } from '../message-utils.js';
 import { userIsMember } from '../thread-utils.js';
 
@@ -24,6 +28,29 @@
   };
 }
 
+function createLeaveThreadSubthreadsUpdates(
+  dmOperation: DMLeaveThreadOperation,
+  threadInfo: ThickRawThreadInfo,
+  viewerID: string,
+  threadInfos: RawThreadInfos,
+): $ReadOnlyArray<ClientUpdateInfo> {
+  const updates = [];
+  for (const thread of values(threadInfos)) {
+    if (thread.parentThreadID !== threadInfo.id) {
+      continue;
+    }
+
+    updates.push({
+      type: updateTypes.DELETE_THREAD,
+      id: uuid.v4(),
+      time: dmOperation.time,
+      threadID: thread.id,
+    });
+  }
+
+  return updates;
+}
+
 const leaveThreadSpec: DMOperationSpec<DMLeaveThreadOperation> = Object.freeze({
   notificationsCreationData: async (dmOperation: DMLeaveThreadOperation) => {
     const messageData = createMessageDataFromDMOperation(dmOperation);
@@ -60,6 +87,12 @@
             time,
             threadID,
           },
+          ...createLeaveThreadSubthreadsUpdates(
+            dmOperation,
+            threadInfo,
+            viewerID,
+            utilities.threadInfos,
+          ),
         ],
       };
     }