diff --git a/app/static/css/app.css b/app/static/css/app.css
index ee8cec7..65df60d 100644
--- a/app/static/css/app.css
+++ b/app/static/css/app.css
@@ -80,6 +80,18 @@ ul {
   box-shadow: var(--shadow);
   padding: 40px 36px;
 }
+@media (max-width: 480px) {
+  .auth-container {
+    padding: 12px;
+  }
+  .auth-card {
+    padding: 28px 20px;
+    border-radius: 12px;
+  }
+  .auth-title {
+    font-size: 20px;
+  }
+}
 
 .auth-logo {
   display: flex;
@@ -1347,39 +1359,300 @@ ul {
 }
 
 /* ── Responsive ──────────────────────────────────────────────────── */
+
+/* Mobile top bar — hidden on tablet+ */
+.mobile-topbar {
+  display: none;
+  align-items: center;
+  gap: 12px;
+  padding: 0 16px;
+  height: 54px;
+  background: var(--color-sidebar-bg);
+  color: #fff;
+  flex-shrink: 0;
+  position: sticky;
+  top: 0;
+  z-index: 50;
+}
+.btn-mobile-menu {
+  font-size: 22px;
+  color: rgba(255, 255, 255, 0.8);
+  background: none;
+  border: none;
+  padding: 4px 6px;
+  cursor: pointer;
+  flex-shrink: 0;
+}
+.btn-mobile-menu:hover {
+  color: #fff;
+}
+.mobile-logo {
+  flex: 1;
+  font-size: 16px;
+  font-weight: 700;
+  color: #fff;
+}
+.btn-fab-mobile {
+  width: 36px;
+  height: 36px;
+  font-size: 20px;
+  flex-shrink: 0;
+}
+
+/* Sidebar backdrop — only active on mobile */
+.sidebar-backdrop {
+  display: none;
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.5);
+  z-index: 89;
+}
+.sidebar-backdrop.open {
+  display: block;
+}
+
+/* ── Phone  (≤ 640px) ─────────────────────────────────────────────── */
 @media (max-width: 640px) {
+  /* Layout: stack topbar above main, no sidebar space in flow */
+  .app-layout {
+    flex-direction: column;
+    height: 100vh;
+    overflow: hidden;
+  }
+
+  /* Topbar always visible */
+  .mobile-topbar {
+    display: flex;
+  }
+
+  /* Sidebar slides in from the left as an overlay */
   .sidebar {
-    width: 72px;
-    overflow-y: visible;
+    position: fixed;
+    top: 0;
+    left: 0;
+    bottom: 0;
+    width: 260px !important; /* always full-width on mobile regardless of collapsed state */
+    z-index: 90;
+    transform: translateX(-100%);
+    transition: transform 0.25s ease;
+    overflow-y: auto;
   }
-  .sidebar-header .logo-text,
-  .sidebar-label {
-    opacity: 0;
-    max-width: 0;
+  .sidebar.mobile-open {
+    transform: translateX(0);
   }
-  .sidebar-section-header {
-    opacity: 0;
-    pointer-events: none;
-    height: 0;
-    padding: 0;
+  /* Show all labels inside the mobile overlay sidebar */
+  .sidebar .sidebar-label {
+    opacity: 1 !important;
+    max-width: none !important;
   }
-  .sidebar-item {
-    justify-content: center;
-    padding: 10px 0;
+  .sidebar .sidebar-section-header {
+    opacity: 1 !important;
+    height: auto !important;
+    padding: 12px 16px 4px !important;
   }
+  .sidebar .sidebar-item {
+    justify-content: flex-start !important;
+    padding: 9px 16px !important;
+    overflow: hidden !important;
+  }
+  .sidebar .sidebar-item::after {
+    display: none !important;
+  }
+  /* Hide desktop collapse toggle on mobile */
+  .btn-sidebar-toggle {
+    display: none;
+  }
+
+  /* Main area fills everything below topbar */
+  .vault-main {
+    flex: 1;
+    overflow: hidden;
+  }
+
+  /* Tighter padding */
   .vault-header {
     padding: 12px 16px;
+    gap: 8px;
+  }
+  .vault-title {
+    font-size: 16px;
   }
   .vault-list {
-    padding: 8px 16px 16px;
+    padding: 8px 12px 80px;
+  } /* bottom pad for FAB */
+  .panel-body {
+    padding: 16px;
+  }
+  .security-dashboard {
+    padding: 12px;
+    gap: 16px;
+  }
+
+  /* Search takes full width, hide sort */
+  .vault-toolbar {
+    gap: 8px;
+  }
+  .search-wrapper {
+    max-width: none;
   }
   .sort-wrapper {
     display: none;
   }
+
+  /* FAB in header is hidden on mobile (use topbar FAB) */
+  .vault-header .btn-fab {
+    display: none;
+  }
+
+  /* Forms */
   .form-row {
     flex-direction: column;
     gap: 0;
   }
+  .modal {
+    padding: 20px;
+  }
+  .modal-actions {
+    flex-wrap: wrap;
+  }
+
+  /* Security stats wrap */
+  .security-summary {
+    padding: 16px;
+    gap: 12px;
+  }
+  .sec-score {
+    font-size: 40px;
+  }
+  .sec-stats {
+    gap: 8px;
+  }
+  .sec-stat {
+    padding: 8px 12px;
+    min-width: 68px;
+  }
+  .sec-stat-num {
+    font-size: 22px;
+  }
+
+  /* Modal: full-screen on phones */
+  .modal-overlay {
+    padding: 0;
+    align-items: flex-end;
+  }
+  .modal {
+    max-width: 100%;
+    max-height: 92vh;
+    border-radius: 16px 16px 0 0;
+    padding: 20px 16px;
+  }
+  .modal-settings {
+    max-width: 100%;
+  }
+  .modal-item {
+    max-width: 100%;
+  }
+
+  /* Detail label stacks above value on small screens */
+  .detail-field {
+    flex-direction: column;
+    gap: 4px;
+  }
+  .detail-label {
+    width: auto;
+  }
+}
+
+/* ── Tablet  (641px – 1024px) ─────────────────────────────────────── */
+@media (min-width: 641px) and (max-width: 1024px) {
+  /* Sidebar always shows as icon rail on tablet — user can expand with toggle */
+  .sidebar:not(.collapsed) {
+    width: 200px;
+  }
+
+  /* Slightly tighter padding */
+  .vault-header {
+    padding: 14px 18px;
+  }
+  .vault-list {
+    padding: 10px 18px 18px;
+  }
+  .panel-body {
+    padding: 16px 18px;
+  }
+  .security-dashboard {
+    padding: 16px;
+    gap: 16px;
+  }
+
+  /* Search a bit narrower */
+  .search-wrapper {
+    max-width: 280px;
+  }
+
+  /* Security stats wrap gracefully */
+  .security-summary {
+    flex-wrap: wrap;
+    gap: 12px;
+  }
+  .sec-stats {
+    flex-wrap: wrap;
+    gap: 10px;
+  }
+
+  /* Form rows stay side-by-side */
+  .form-row {
+    flex-direction: row;
+  }
+
+  /* Modal fits tablet width */
+  .modal {
+    max-width: 460px;
+  }
+}
+
+/* ── Laptop  (1025px – 1440px) ───────────────────────────────────── */
+@media (min-width: 1025px) and (max-width: 1440px) {
+  /* Full sidebar, comfortable padding */
+  .vault-header {
+    padding: 16px 24px;
+  }
+  .vault-list {
+    padding: 12px 24px 24px;
+  }
+  .security-dashboard {
+    padding: 20px 24px;
+  }
+}
+
+/* ── Large PC  (> 1440px) ────────────────────────────────────────── */
+@media (min-width: 1441px) {
+  /* Wider sidebar, more generous spacing */
+  .sidebar:not(.collapsed) {
+    width: 260px;
+  }
+  .vault-header {
+    padding: 18px 32px;
+  }
+  .vault-list {
+    padding: 16px 32px 32px;
+  }
+  .panel-body {
+    padding: 24px 32px;
+  }
+  .security-dashboard {
+    padding: 24px 32px;
+    gap: 28px;
+  }
+  .search-wrapper {
+    max-width: 440px;
+  }
+  .modal {
+    max-width: 520px;
+  }
+  .modal-settings {
+    max-width: 580px;
+  }
 }
 
 /* ── Settings — new sections (Phase 6) ──────────────────────────── */
diff --git a/app/static/js/vault.js b/app/static/js/vault.js
index 3fc5a5d..665804c 100644
--- a/app/static/js/vault.js
+++ b/app/static/js/vault.js
@@ -2645,6 +2645,34 @@ const Vault = (() => {
       });
     }
 
+    // Mobile sidebar: hamburger opens overlay, backdrop/item-click closes it
+    const backdrop = document.getElementById("sidebar-backdrop");
+    const mobileMenuBtn = document.getElementById("btn-mobile-menu");
+    function openMobileSidebar() {
+      sidebar?.classList.add("mobile-open");
+      backdrop?.classList.add("open");
+      document.body.style.overflow = "hidden";
+    }
+    function closeMobileSidebar() {
+      sidebar?.classList.remove("mobile-open");
+      backdrop?.classList.remove("open");
+      document.body.style.overflow = "";
+    }
+    mobileMenuBtn?.addEventListener("click", openMobileSidebar);
+    backdrop?.addEventListener("click", closeMobileSidebar);
+    // Close mobile sidebar whenever a nav item is clicked
+    document.querySelectorAll(".sidebar-item").forEach((item) => {
+      item.addEventListener("click", () => {
+        if (window.innerWidth <= 640) closeMobileSidebar();
+      });
+    });
+    // Mobile FAB mirrors the desktop add-item button
+    document
+      .getElementById("btn-add-item-mobile")
+      ?.addEventListener("click", () => {
+        document.getElementById("btn-add-item")?.click();
+      });
+
     if (!VaultSession.getKey()) {
       showUnlockOverlay();
     } else {
diff --git a/app/templates/vault/index.html b/app/templates/vault/index.html
index cf086b8..cfd8478 100644
--- a/app/templates/vault/index.html
+++ b/app/templates/vault/index.html
@@ -1,6 +1,24 @@
 {% extends "base.html" %} {% block title %}My Vault — PassKeeper{% endblock %}
 {% block body_class %}vault-page{% endblock %} {% block body %}
 <div class="app-layout">
+  <!-- ── Mobile top bar (phones only) ────────────────────────────── -->
+  <div class="mobile-topbar" id="mobile-topbar">
+    <button class="btn-mobile-menu" id="btn-mobile-menu" aria-label="Open menu">
+      ☰
+    </button>
+    <span class="mobile-logo">🔒 PassKeeper</span>
+    <button
+      class="btn-fab btn-fab-mobile"
+      id="btn-add-item-mobile"
+      title="Add item"
+    >
+      +
+    </button>
+  </div>
+
+  <!-- ── Sidebar backdrop (mobile) ───────────────────────────────── -->
+  <div class="sidebar-backdrop" id="sidebar-backdrop"></div>
+
   <!-- ── Sidebar ─────────────────────────────────────────────────── -->
   <aside class="sidebar" id="sidebar">
     <div class="sidebar-header">