diff --git a/lib/handlers/dm-activity-handler.js b/lib/handlers/dm-activity-handler.js
new file mode 100644
--- /dev/null
+++ b/lib/handlers/dm-activity-handler.js
@@ -0,0 +1,107 @@
+// @flow
+import invariant from 'invariant';
+import * as React from 'react';
+
+import { updateActivityActionTypes } from '../actions/activity-actions.js';
+import {
+  type OutboundDMOperationSpecification,
+  dmOperationSpecificationTypes,
+} from '../shared/dm-ops/dm-op-utils.js';
+import { useProcessAndSendDMOperation } from '../shared/dm-ops/process-dm-ops.js';
+import { getMostRecentNonLocalMessageID } from '../shared/message-utils.js';
+import { threadIsPending } from '../shared/thread-utils.js';
+import type { ActivityUpdateSuccessPayload } from '../types/activity-types.js';
+import type { DMChangeThreadReadStatusOperation } from '../types/dm-ops.js';
+import type { RawThreadInfo } from '../types/minimally-encoded-thread-permissions-types.js';
+import { threadTypeIsThick } from '../types/thread-types-enum.js';
+import { useDispatchActionPromise } from '../utils/redux-promise-utils.js';
+import { useSelector } from '../utils/redux-utils.js';
+
+function useUpdateDMActivity(): (
+  viewerID: string,
+  activeThreadInfo: RawThreadInfo,
+) => Promise<ActivityUpdateSuccessPayload> {
+  const processAndSendDMOperation = useProcessAndSendDMOperation();
+  return React.useCallback(
+    async (viewerID: string, activeThreadInfo: RawThreadInfo) => {
+      invariant(
+        threadTypeIsThick(activeThreadInfo.type),
+        'thread must be thick',
+      );
+      const op: DMChangeThreadReadStatusOperation = {
+        type: 'change_thread_read_status',
+        time: Date.now(),
+        threadID: activeThreadInfo.id,
+        creatorID: viewerID,
+        unread: false,
+      };
+
+      const opSpecification: OutboundDMOperationSpecification = {
+        type: dmOperationSpecificationTypes.OUTBOUND,
+        op,
+        recipients: { type: 'self_devices' },
+      };
+
+      await processAndSendDMOperation(opSpecification);
+      return { activityUpdates: {}, result: { unfocusedToUnread: [] } };
+    },
+    [processAndSendDMOperation],
+  );
+}
+
+function useDMActivityHandler(activeThread: ?string): void {
+  const activeThreadInfo = useSelector(state =>
+    activeThread ? state.threadStore.threadInfos[activeThread] : null,
+  );
+  const activeThreadLatestMessage = useSelector(state =>
+    activeThread
+      ? getMostRecentNonLocalMessageID(activeThread, state.messageStore)
+      : null,
+  );
+  const processAndSendDMOperation = useProcessAndSendDMOperation();
+
+  const prevActiveThreadRef = React.useRef<?string>();
+  const prevActiveThreadLatestMessageRef = React.useRef<?string>();
+
+  const viewerID = useSelector(
+    state => state.currentUserInfo && state.currentUserInfo.id,
+  );
+  const updateDMActivity = useUpdateDMActivity();
+  const dispatchActionPromise = useDispatchActionPromise();
+
+  React.useEffect(() => {
+    const prevActiveThread = prevActiveThreadRef.current;
+    const prevActiveThreadLatestMessage =
+      prevActiveThreadLatestMessageRef.current;
+
+    prevActiveThreadRef.current = activeThread;
+    prevActiveThreadLatestMessageRef.current = activeThreadLatestMessage;
+
+    if (
+      !viewerID ||
+      !activeThread ||
+      !activeThreadInfo ||
+      !threadTypeIsThick(activeThreadInfo.type) ||
+      threadIsPending(activeThread) ||
+      (activeThread === prevActiveThread &&
+        activeThreadLatestMessage === prevActiveThreadLatestMessage)
+    ) {
+      return;
+    }
+
+    void dispatchActionPromise(
+      updateActivityActionTypes,
+      updateDMActivity(viewerID, activeThreadInfo),
+    );
+  }, [
+    updateDMActivity,
+    dispatchActionPromise,
+    activeThread,
+    viewerID,
+    processAndSendDMOperation,
+    activeThreadInfo,
+    activeThreadLatestMessage,
+  ]);
+}
+
+export default useDMActivityHandler;
diff --git a/native/components/dm-activity-handler.react.js b/native/components/dm-activity-handler.react.js
new file mode 100644
--- /dev/null
+++ b/native/components/dm-activity-handler.react.js
@@ -0,0 +1,27 @@
+// @flow
+import * as React from 'react';
+
+import useDMActivityHandler from 'lib/handlers/dm-activity-handler.js';
+import { isLoggedIn } from 'lib/selectors/user-selectors.js';
+
+import { activeMessageListSelector } from '../navigation/nav-selectors.js';
+import { NavContext } from '../navigation/navigation-context.js';
+import { useSelector } from '../redux/redux-utils.js';
+
+function DMActivityHandler(): React.Node {
+  const active = useSelector(
+    state => isLoggedIn(state) && state.lifecycleState !== 'background',
+  );
+  const navContext = React.useContext(NavContext);
+  const activeThread = React.useMemo(() => {
+    if (!active) {
+      return null;
+    }
+    return activeMessageListSelector(navContext);
+  }, [active, navContext]);
+
+  useDMActivityHandler(activeThread);
+  return null;
+}
+
+export default DMActivityHandler;
diff --git a/native/root.react.js b/native/root.react.js
--- a/native/root.react.js
+++ b/native/root.react.js
@@ -54,6 +54,7 @@
 import { AutoJoinCommunityHandler } from './components/auto-join-community-handler.react.js';
 import BackgroundIdentityLoginHandler from './components/background-identity-login-handler.react.js';
 import ConnectFarcasterAlertHandler from './components/connect-farcaster-alert-handler.react.js';
+import DMActivityHandler from './components/dm-activity-handler.react.js';
 import { FeatureFlagsProvider } from './components/feature-flags-provider.react.js';
 import { NUXTipsContextProvider } from './components/nux-tips-context.react.js';
 import PersistedStateGate from './components/persisted-state-gate.js';
@@ -373,6 +374,7 @@
                                                         detectUnsupervisedBackgroundRef
                                                       }
                                                     />
+                                                    <DMActivityHandler />
                                                     <VersionSupportedChecker />
                                                     <PlatformDetailsSynchronizer />
                                                     <BackgroundIdentityLoginHandler />
diff --git a/web/app.react.js b/web/app.react.js
--- a/web/app.react.js
+++ b/web/app.react.js
@@ -55,6 +55,7 @@
 import { MemberListSidebarProvider } from './chat/member-list-sidebar/member-list-sidebar-provider.react.js';
 import { AutoJoinCommunityHandler } from './components/auto-join-community-handler.react.js';
 import CommunitiesRefresher from './components/communities-refresher.react.js';
+import DMActivityHandler from './components/dm-activity-handler.react.js';
 import LogOutIfMissingCSATHandler from './components/log-out-if-missing-csat-handler.react.js';
 import NavigationArrows from './components/navigation-arrows.react.js';
 import MinVersionHandler from './components/version-handler.react.js';
@@ -256,6 +257,7 @@
                       <TunnelbrokerDeviceTokenHandler />
                       <FarcasterDataHandler />
                       <AutoJoinCommunityHandler />
+                      <DMActivityHandler />
                       {content}
                     </ChatMentionContextProvider>
                   </MessageSearchStateProvider>
diff --git a/web/components/dm-activity-handler.react.js b/web/components/dm-activity-handler.react.js
new file mode 100644
--- /dev/null
+++ b/web/components/dm-activity-handler.react.js
@@ -0,0 +1,27 @@
+// @flow
+import * as React from 'react';
+
+import useDMActivityHandler from 'lib/handlers/dm-activity-handler.js';
+import { isLoggedIn } from 'lib/selectors/user-selectors.js';
+
+import { useSelector } from '../redux/redux-utils.js';
+import { activeThreadSelector } from '../selectors/nav-selectors.js';
+
+function DMActivityHandler(): React.Node {
+  const active = useSelector(
+    state => isLoggedIn(state) && state.lifecycleState !== 'background',
+  );
+  const reduxActiveThread = useSelector(activeThreadSelector);
+  const windowActive = useSelector(state => state.windowActive);
+  const activeThread = React.useMemo(() => {
+    if (!active || !windowActive) {
+      return null;
+    }
+    return reduxActiveThread;
+  }, [active, windowActive, reduxActiveThread]);
+
+  useDMActivityHandler(activeThread);
+  return null;
+}
+
+export default DMActivityHandler;