diff --git a/apps/web/src/app/(authenticated)/layout.tsx b/apps/web/src/app/(authenticated)/layout.tsx
index 68cc7a3..9099d6a 100644
--- a/apps/web/src/app/(authenticated)/layout.tsx
+++ b/apps/web/src/app/(authenticated)/layout.tsx
@@ -27,11 +27,16 @@ interface AppShellProps {
 }
 
 function AppShell({ children }: AppShellProps): React.JSX.Element {
-  const { collapsed } = useSidebar();
+  const { collapsed, isMobile } = useSidebar();
+
+  // On tablet (md–lg), hide sidebar from the grid when the sidebar is collapsed.
+  // On mobile, the sidebar is fixed-position so the grid is always single-column.
+  const sidebarHidden = !isMobile && collapsed;
 
   return (
     <div
       className="app-shell"
+      data-sidebar-hidden={sidebarHidden ? "true" : undefined}
       style={
         {
           "--sidebar-w": collapsed ? SIDEBAR_COLLAPSED_WIDTH : SIDEBAR_EXPANDED_WIDTH,
diff --git a/apps/web/src/app/globals.css b/apps/web/src/app/globals.css
index 9206789..f19b900 100644
--- a/apps/web/src/app/globals.css
+++ b/apps/web/src/app/globals.css
@@ -249,6 +249,59 @@ body::before {
   position: relative;
 }
 
+/* -----------------------------------------------------------------------------
+   Responsive App Shell — Mobile (< 768px): single-column, sidebar as overlay
+   ----------------------------------------------------------------------------- */
+@media (max-width: 767px) {
+  .app-shell {
+    grid-template-columns: 1fr;
+  }
+
+  .app-sidebar {
+    position: fixed;
+    left: 0;
+    top: var(--topbar-h);
+    bottom: 0;
+    width: 240px;
+    z-index: 150;
+    transform: translateX(-100%);
+    transition: transform 0.2s ease;
+  }
+
+  .app-sidebar[data-mobile-open="true"] {
+    transform: translateX(0);
+  }
+
+  .app-main {
+    grid-column: 1;
+  }
+
+  .app-header {
+    grid-column: 1;
+  }
+}
+
+/* -----------------------------------------------------------------------------
+   Responsive App Shell — Tablet (768px–1023px): sidebar toggleable, pushes content
+   ----------------------------------------------------------------------------- */
+@media (min-width: 768px) and (max-width: 1023px) {
+  .app-shell[data-sidebar-hidden="true"] {
+    grid-template-columns: 1fr;
+  }
+
+  .app-shell[data-sidebar-hidden="true"] .app-sidebar {
+    display: none;
+  }
+
+  .app-shell[data-sidebar-hidden="true"] .app-main {
+    grid-column: 1;
+  }
+
+  .app-shell[data-sidebar-hidden="true"] .app-header {
+    grid-column: 1;
+  }
+}
+
 /* -----------------------------------------------------------------------------
    Typography Utilities
    ----------------------------------------------------------------------------- */
diff --git a/apps/web/src/components/layout/AppHeader.tsx b/apps/web/src/components/layout/AppHeader.tsx
index 1002bd7..0c6f28b 100644
--- a/apps/web/src/components/layout/AppHeader.tsx
+++ b/apps/web/src/components/layout/AppHeader.tsx
@@ -5,6 +5,7 @@ import Link from "next/link";
 import { usePathname } from "next/navigation";
 import { useAuth } from "@/lib/auth/auth-context";
 import { ThemeToggle } from "./ThemeToggle";
+import { useSidebar } from "./SidebarContext";
 
 /**
  * Full-width application header (topbar).
@@ -17,6 +18,7 @@ import { ThemeToggle } from "./ThemeToggle";
  */
 export function AppHeader(): React.JSX.Element {
   const { user, signOut } = useAuth();
+  const { isMobile, mobileOpen, setMobileOpen, toggleCollapsed } = useSidebar();
   const pathname = usePathname();
   const [dropdownOpen, setDropdownOpen] = useState(false);
   const [searchFocused, setSearchFocused] = useState(false);
@@ -58,8 +60,59 @@ export function AppHeader(): React.JSX.Element {
       ? (user.email[0] ?? "?").toUpperCase()
       : "?";
 
+  const handleHamburgerClick = useCallback((): void => {
+    if (isMobile) {
+      setMobileOpen(!mobileOpen);
+    } else {
+      toggleCollapsed();
+    }
+  }, [isMobile, mobileOpen, setMobileOpen, toggleCollapsed]);
+
   return (
     <header className="app-header">
+      {/* ── Hamburger — visible below lg ── */}
+      <button
+        type="button"
+        className="lg:hidden"
+        onClick={handleHamburgerClick}
+        aria-label={mobileOpen ? "Close navigation menu" : "Open navigation menu"}
+        aria-expanded={mobileOpen}
+        aria-controls="app-sidebar"
+        style={{
+          width: 34,
+          height: 34,
+          borderRadius: 6,
+          display: "flex",
+          alignItems: "center",
+          justifyContent: "center",
+          color: "var(--muted)",
+          background: "none",
+          border: "none",
+          cursor: "pointer",
+          flexShrink: 0,
+        }}
+        onMouseEnter={(e) => {
+          (e.currentTarget as HTMLButtonElement).style.background = "var(--surface)";
+          (e.currentTarget as HTMLButtonElement).style.color = "var(--text)";
+        }}
+        onMouseLeave={(e) => {
+          (e.currentTarget as HTMLButtonElement).style.background = "none";
+          (e.currentTarget as HTMLButtonElement).style.color = "var(--muted)";
+        }}
+      >
+        <svg
+          width="16"
+          height="16"
+          viewBox="0 0 16 16"
+          fill="none"
+          stroke="currentColor"
+          strokeWidth="1.5"
+          aria-hidden="true"
+        >
+          <path d="M2 4h12M2 8h12M2 12h12" />
+        </svg>
+      </button>
+
       {/* ── Brand / Logo ── */}
       <Link
         href="/"
diff --git a/apps/web/src/components/layout/AppSidebar.tsx b/apps/web/src/components/layout/AppSidebar.tsx
index ef3ecb3..eca49de 100644
--- a/apps/web/src/components/layout/AppSidebar.tsx
+++ b/apps/web/src/components/layout/AppSidebar.tsx
@@ -641,70 +641,91 @@ function CollapseToggle({ collapsed, onToggle }: CollapseToggleProps): React.JSX
  */
 export function AppSidebar(): React.JSX.Element {
   const pathname = usePathname();
-  const { collapsed, toggleCollapsed } = useSidebar();
+  const { collapsed, toggleCollapsed, mobileOpen, setMobileOpen, isMobile } = useSidebar();
 
   return (
-    <aside
-      className="app-sidebar"
-      data-collapsed={collapsed ? "true" : undefined}
-      aria-label="Application navigation"
-    >
-      {/* Sidebar body — scrollable nav area */}
-      <nav
-        style={{
-          flex: 1,
-          overflowY: "auto",
-          overflowX: "hidden",
-          padding: "10px 10px",
-        }}
-        aria-label="Main navigation"
+    <>
+      {/* Mobile backdrop — rendered behind the sidebar when open on mobile */}
+      {isMobile && mobileOpen && (
+        <div
+          aria-hidden="true"
+          onClick={() => {
+            setMobileOpen(false);
+          }}
+          style={{
+            position: "fixed",
+            inset: 0,
+            top: "var(--topbar-h)",
+            background: "rgba(0,0,0,0.5)",
+            zIndex: 140,
+          }}
+        />
+      )}
+
+      <aside
+        id="app-sidebar"
+        className="app-sidebar"
+        data-collapsed={collapsed ? "true" : undefined}
+        data-mobile-open={mobileOpen ? "true" : undefined}
+        aria-label="Application navigation"
       >
-        {NAV_GROUPS.map((group) => (
-          <div key={group.label} style={{ marginBottom: "18px" }}>
-            {/* Group label — hidden when collapsed */}
-            {!collapsed && (
-              <p
-                style={{
-                  fontSize: "0.67rem",
-                  fontWeight: 600,
-                  textTransform: "uppercase",
-                  letterSpacing: "0.09em",
-                  color: "var(--muted)",
-                  padding: "0 10px",
-                  marginBottom: "4px",
-                  userSelect: "none",
-                }}
-              >
-                {group.label}
-              </p>
-            )}
+        {/* Sidebar body — scrollable nav area */}
+        <nav
+          style={{
+            flex: 1,
+            overflowY: "auto",
+            overflowX: "hidden",
+            padding: "10px 10px",
+          }}
+          aria-label="Main navigation"
+        >
+          {NAV_GROUPS.map((group) => (
+            <div key={group.label} style={{ marginBottom: "18px" }}>
+              {/* Group label — hidden when collapsed */}
+              {!collapsed && (
+                <p
+                  style={{
+                    fontSize: "0.67rem",
+                    fontWeight: 600,
+                    textTransform: "uppercase",
+                    letterSpacing: "0.09em",
+                    color: "var(--muted)",
+                    padding: "0 10px",
+                    marginBottom: "4px",
+                    userSelect: "none",
+                  }}
+                >
+                  {group.label}
+                </p>
+              )}
 
-            {/* Nav items */}
-            <ul style={{ listStyle: "none", margin: 0, padding: 0 }}>
-              {group.items.map((item) => {
-                const isActive =
-                  item.href === "/"
-                    ? pathname === "/"
-                    : item.href.startsWith("#")
-                      ? false
-                      : pathname === item.href || pathname.startsWith(item.href + "/");
+              {/* Nav items */}
+              <ul style={{ listStyle: "none", margin: 0, padding: 0 }}>
+                {group.items.map((item) => {
+                  const isActive =
+                    item.href === "/"
+                      ? pathname === "/"
+                      : item.href.startsWith("#")
+                        ? false
+                        : pathname === item.href || pathname.startsWith(item.href + "/");
 
-                return (
-                  <li key={item.href}>
-                    <NavItem item={item} isActive={isActive} collapsed={collapsed} />
-                  </li>
-                );
-              })}
-            </ul>
-          </div>
-        ))}
+                  return (
+                    <li key={item.href}>
+                      <NavItem item={item} isActive={isActive} collapsed={collapsed} />
+                    </li>
+                  );
+                })}
+              </ul>
+            </div>
+          ))}
 
-        {/* Collapse toggle — anchored at bottom of nav */}
-        <CollapseToggle collapsed={collapsed} onToggle={toggleCollapsed} />
-      </nav>
+          {/* Collapse toggle — anchored at bottom of nav */}
+          <CollapseToggle collapsed={collapsed} onToggle={toggleCollapsed} />
+        </nav>
 
-      {/* User card footer */}
-      <UserCard collapsed={collapsed} />
-    </aside>
+        {/* User card footer */}
+        <UserCard collapsed={collapsed} />
+      </aside>
+    </>
   );
 }
diff --git a/apps/web/src/components/layout/SidebarContext.tsx b/apps/web/src/components/layout/SidebarContext.tsx
index ca83270..7d8eaf6 100644
--- a/apps/web/src/components/layout/SidebarContext.tsx
+++ b/apps/web/src/components/layout/SidebarContext.tsx
@@ -1,22 +1,57 @@
 "use client";
 
-import { createContext, useContext, useState, useCallback, type ReactNode } from "react";
+import { createContext, useContext, useState, useCallback, useEffect, type ReactNode } from "react";
 
 interface SidebarContextValue {
   collapsed: boolean;
   toggleCollapsed: () => void;
+  mobileOpen: boolean;
+  setMobileOpen: (open: boolean) => void;
+  isMobile: boolean;
 }
 
 const SidebarContext = createContext<SidebarContextValue | undefined>(undefined);
 
+/** Breakpoint below which we treat the viewport as "mobile" (matches CSS max-width: 767px). */
+const MOBILE_MAX_WIDTH = 767;
+
 export function SidebarProvider({ children }: { children: ReactNode }): React.JSX.Element {
   const [collapsed, setCollapsed] = useState(false);
+  const [mobileOpen, setMobileOpen] = useState(false);
+  const [isMobile, setIsMobile] = useState(false);
+
+  // Initialise and track mobile breakpoint using matchMedia
+  useEffect((): (() => void) => {
+    const mql = window.matchMedia(`(max-width: ${String(MOBILE_MAX_WIDTH)}px)`);
+
+    const handleChange = (e: MediaQueryListEvent): void => {
+      setIsMobile(e.matches);
+      // Close mobile sidebar when viewport grows out of mobile range
+      if (!e.matches) {
+        setMobileOpen(false);
+      }
+    };
+
+    // Set initial value synchronously
+    setIsMobile(mql.matches);
+
+    mql.addEventListener("change", handleChange);
+    return (): void => {
+      mql.removeEventListener("change", handleChange);
+    };
+  }, []);
 
   const toggleCollapsed = useCallback((): void => {
     setCollapsed((prev) => !prev);
   }, []);
 
-  const value: SidebarContextValue = { collapsed, toggleCollapsed };
+  const value: SidebarContextValue = {
+    collapsed,
+    toggleCollapsed,
+    mobileOpen,
+    setMobileOpen,
+    isMobile,
+  };
 
   return <SidebarContext.Provider value={value}>{children}</SidebarContext.Provider>;
 }