diff --git a/index.html b/index.html
index 3cf5573..054bad8 100644
--- a/index.html
+++ b/index.html
@@ -566,6 +566,19 @@
                         <option value="60">60</option>
                     </select>
                 </div>
+                <div class="setting-row">
+                    <span>
+                        Lock vault when Windows locks or sleeps
+                        <small class="setting-hint">
+                            With Quick Unlock enabled the vault stays unlocked
+                            anyway — Windows sign-in already gates access.
+                        </small>
+                    </span>
+                    <label class="toggle">
+                        <input type="checkbox" id="settingLockOnSystemLock">
+                        <span class="toggle-slider"></span>
+                    </label>
+                </div>
                 <div class="setting-row">
                     <span>Ask before moving to trash</span>
                     <label class="toggle">
diff --git a/js/app.js b/js/app.js
index 92bea24..579ee6d 100644
--- a/js/app.js
+++ b/js/app.js
@@ -86,6 +86,9 @@ const Bridge = (() => {
         // re-locking is redundant — we just stay unlocked and the user is
         // back where they left off when they return.
         onSystemLock() {
+            // Explicit user opt-out (Settings → Security). Default ON keeps
+            // the historical behavior below.
+            if (state.lockOnSystemLock === false) return;
             if (state.quickUnlockEnabled) {
                 if (typeof toast === 'function')
                     toast('System lock — vault kept unlocked (quick unlock active)');
@@ -650,6 +653,7 @@ const state = {
     autofillEnabled: localStorage.getItem('autofillEnabled') !== '0', // default ON
     autofillClearField: localStorage.getItem('autofillClearField') !== '0', // default ON
     clipboardClearSeconds: parseInt(localStorage.getItem('clipboardClearSeconds') ?? '30', 10), // 0 = never
+    lockOnSystemLock: localStorage.getItem('lockOnSystemLock') !== '0', // default ON
     autofillFailBalloon: localStorage.getItem('autofillFailBalloon') !== '0', // default ON
     // Hotkey combos. Each combo = { ctrl, shift, alt, win, key }.
     // key is the uppercase character or VK label ('A'..'Z', '0'..'9',
@@ -6398,6 +6402,7 @@ function openSettings() {
     $('#settingTrashPurge').value = String(state.trashAutoPurgeDays || 0);
     $('#settingPasswordExpiry').value = String(state.passwordExpiryDays || 0);
     $('#settingClipboardClear').value = String(state.clipboardClearSeconds ?? 30);
+    $('#settingLockOnSystemLock').checked = state.lockOnSystemLock !== false;
     $('#settingEditorPosition').value = state.editorPosition || 'right';
     $('#settingConfirmUnsaved').checked = state.confirmOnUnsaved !== false;
     // PIN unlock — only meaningful when DPAPI is available.
@@ -6796,6 +6801,8 @@ const SYNCED_SETTING_KEYS = [
     'autofillFailBalloon',
     // Seconds before a copied secret is auto-cleared (0 = never). Default 30.
     'clipboardClearSeconds',
+    // Lock the vault when Windows locks / sleeps (default ON).
+    'lockOnSystemLock',
 ];
 
 // Sets `data-editor-position` on <body> so CSS can swap the slideover
@@ -6885,6 +6892,9 @@ async function loadServerSettings() {
                 case 'clipboardClearSeconds':
                     localStorage.setItem('clipboardClearSeconds', String(v));
                     break;
+                case 'lockOnSystemLock':
+                    localStorage.setItem('lockOnSystemLock', v ? '1' : '0');
+                    break;
             }
         });
         // Apply visual settings immediately.
@@ -7586,6 +7596,13 @@ async function init() {
         saveServerSettings();
         toast(n === 0 ? 'Clipboard auto-clear disabled' : 'Copied secrets clear after ' + n + 's');
     });
+    $('#settingLockOnSystemLock').addEventListener('change', e => {
+        state.lockOnSystemLock = e.target.checked;
+        localStorage.setItem('lockOnSystemLock', e.target.checked ? '1' : '0');
+        saveServerSettings();
+        toast(e.target.checked ? 'Vault will lock when Windows locks'
+                               : 'Vault stays unlocked when Windows locks');
+    });
     $('#settingPasswordExpiry').addEventListener('change', e => {
         const n = parseInt(e.target.value, 10) || 0;
         state.passwordExpiryDays = n;
