diff --git a/lib/reducers/master-reducer.js b/lib/reducers/master-reducer.js
--- a/lib/reducers/master-reducer.js
+++ b/lib/reducers/master-reducer.js
@@ -149,6 +149,7 @@
       threadActivityStore: reduceThreadActivity(
         state.threadActivityStore,
         action,
+        threadInfos,
       ),
     },
     storeOperations: {
diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -1806,4 +1806,8 @@
   };
 }
 
-export { freshMessageStore, reduceMessageStore };
+export {
+  freshMessageStore,
+  reduceMessageStore,
+  ensureRealizedThreadIDIsUsedWhenPossible,
+};
diff --git a/lib/reducers/thread-activity-reducer.js b/lib/reducers/thread-activity-reducer.js
--- a/lib/reducers/thread-activity-reducer.js
+++ b/lib/reducers/thread-activity-reducer.js
@@ -1,5 +1,11 @@
 // @flow
 
+import { ensureRealizedThreadIDIsUsedWhenPossible } from './message-reducer.js';
+import {
+  sendMultimediaMessageActionTypes,
+  sendReactionMessageActionTypes,
+  sendTextMessageActionTypes,
+} from '../actions/message-actions.js';
 import {
   logOutActionTypes,
   deleteAccountActionTypes,
@@ -7,11 +13,13 @@
 import type { BaseAction } from '../types/redux-types.js';
 import type { ThreadActivityStore } from '../types/thread-activity-types.js';
 import { updateThreadLastNavigatedActionType } from '../types/thread-activity-types.js';
+import type { RawThreadInfo } from '../types/thread-types.js';
 import { setNewSessionActionType } from '../utils/action-utils.js';
 
 function reduceThreadActivity(
   state: ThreadActivityStore,
   action: BaseAction,
+  newThreadInfos: { +[id: string]: RawThreadInfo },
 ): ThreadActivityStore {
   if (action.type === updateThreadLastNavigatedActionType) {
     const { threadID, time } = action.payload;
@@ -23,6 +31,25 @@
       },
     };
     return updatedThreadActivityStore;
+  } else if (
+    action.type === sendTextMessageActionTypes.started ||
+    action.type === sendMultimediaMessageActionTypes.started ||
+    action.type === sendReactionMessageActionTypes.started
+  ) {
+    const payload = ensureRealizedThreadIDIsUsedWhenPossible(
+      action.payload,
+      newThreadInfos,
+    );
+    const { threadID } = payload;
+    const now = Date.now();
+
+    return {
+      ...state,
+      [threadID]: {
+        lastNavigatedTo: state[threadID]?.lastNavigatedTo ?? now,
+        lastPruned: state[threadID]?.lastPruned ?? now,
+      },
+    };
   } else if (
     action.type === logOutActionTypes.success ||
     action.type === deleteAccountActionTypes.success ||
diff --git a/lib/reducers/thread-activity-reducer.test.js b/lib/reducers/thread-activity-reducer.test.js
--- a/lib/reducers/thread-activity-reducer.test.js
+++ b/lib/reducers/thread-activity-reducer.test.js
@@ -34,7 +34,7 @@
         lastPruned: 1639522314170,
       },
     };
-    const result = reduceThreadActivity(initialState, action);
+    const result = reduceThreadActivity(initialState, action, {});
     expect(result).toEqual(expectedState);
   });
 
@@ -63,7 +63,7 @@
         lastPruned: 1639522314170,
       },
     };
-    const result = reduceThreadActivity(initialState, action);
+    const result = reduceThreadActivity(initialState, action, {});
     expect(result).toEqual(expectedState);
   });
 });