diff --git a/script/qa/cleanup-proj16-testdata.php b/script/qa/cleanup-proj16-testdata.php
new file mode 100644
index 0000000..413326d
--- /dev/null
+++ b/script/qa/cleanup-proj16-testdata.php
@@ -0,0 +1,84 @@
+<?php
+
+/**
+ * Cleanup helper for PROJ-16 QA seed data on shared environments.
+ *
+ * Usage:
+ *   RAILS_ENV=production php script/qa/cleanup-proj16-testdata.php --dry-run
+ *   PROJ16_CLEANUP_TOKEN=your-secret RAILS_ENV=production php script/qa/cleanup-proj16-testdata.php --token=your-secret
+ */
+
+require dirname(__DIR__, 2) . '/config/boot.php';
+
+$dryRun = in_array('--dry-run', $argv, true);
+$runtimeToken = null;
+$dmailCondition = '(from_id IN (?) OR to_id IN (?)) AND title LIKE ?';
+
+foreach ($argv as $arg) {
+    if (str_starts_with($arg, '--token=')) {
+        $runtimeToken = substr($arg, 8);
+    }
+}
+
+if (!$dryRun) {
+    $requiredToken = getenv('PROJ16_CLEANUP_TOKEN');
+
+    if ($requiredToken === false || $requiredToken === '' || $runtimeToken === null || !hash_equals($requiredToken, $runtimeToken)) {
+        fwrite(STDERR, "Refusing destructive cleanup: set PROJ16_CLEANUP_TOKEN and pass matching --token=<value>." . PHP_EOL);
+        exit(2);
+    }
+}
+
+$targetNames = ['qa_proj16_sender', 'qa_proj16_recipient'];
+
+$users = User::where('name IN (?, ?)', $targetNames[0], $targetNames[1])->take();
+$userIds = [];
+
+foreach ($users as $user) {
+    $userIds[] = (int)$user->id;
+}
+
+if (empty($userIds)) {
+    echo "No PROJ-16 test users found (qa_proj16_sender/qa_proj16_recipient)." . PHP_EOL;
+    exit(0);
+}
+
+$summary = [
+    'user_ids' => $userIds,
+    'user_count' => User::where('id IN (?)', $userIds)->count(),
+    'dmail_count' => Dmail::where(
+        $dmailCondition,
+        $userIds,
+        $userIds,
+        'PROJ16 QA%'
+    )->count(),
+];
+
+echo "PROJ-16 cleanup summary:" . PHP_EOL;
+echo json_encode($summary, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES) . PHP_EOL;
+
+if ($dryRun) {
+    echo "Dry run only, no data was deleted." . PHP_EOL;
+    exit(0);
+}
+
+try {
+    User::transaction(function() use ($userIds, $dmailCondition) {
+        // Remove QA dmails first to avoid orphaned rows.
+        Dmail::destroyAll($dmailCondition, $userIds, $userIds, 'PROJ16 QA%');
+
+        // Remove user-related rows if present in this environment.
+        foreach (['UserBlacklistedTag', 'UserLog', 'PostVote', 'Favorite', 'Ban'] as $optionalModelClass) {
+            if (class_exists($optionalModelClass)) {
+                $optionalModelClass::destroyAll('user_id IN (?)', $userIds);
+            }
+        }
+
+        User::destroyAll('id IN (?)', $userIds);
+    });
+} catch (Throwable $e) {
+    fwrite(STDERR, "Cleanup failed: " . $e->getMessage() . PHP_EOL);
+    exit(1);
+}
+
+echo "Cleanup completed." . PHP_EOL;