diff --git a/templates/index.html b/templates/index.html
index 249f2b4..d3877d0 100644
--- a/templates/index.html
+++ b/templates/index.html
@@ -4162,6 +4162,9 @@
             // Initialize dropdown nav active state
             updateDropdownActiveState();
 
+            // Restore nav group open/closed state from localStorage
+            initNavGroupState();
+
             // Start SDR device status polling
             startSdrStatusPolling();
 
@@ -4189,6 +4192,45 @@
             if (!isOpen) {
                 dropdown.classList.add('open');
             }
+            saveNavGroupState();
+        }
+
+        function initNavGroupState() {
+            const NAV_STATE_KEY = 'intercept_nav_groups';
+            let savedState = {};
+            try {
+                savedState = JSON.parse(localStorage.getItem(NAV_STATE_KEY) || '{}');
+            } catch (e) {
+                savedState = {};
+            }
+
+            document.querySelectorAll('.mode-nav-dropdown[data-group]').forEach(dropdown => {
+                const group = dropdown.dataset.group;
+                // If saved state says closed AND this group has no active item, close it
+                if (savedState[group] === false) {
+                    const hasActive = dropdown.classList.contains('has-active');
+                    if (!hasActive) {
+                        dropdown.classList.remove('open');
+                        const btn = dropdown.querySelector('.mode-nav-dropdown-btn');
+                        if (btn) btn.setAttribute('aria-expanded', 'false');
+                    }
+                } else if (savedState[group] === true) {
+                    dropdown.classList.add('open');
+                    const btn = dropdown.querySelector('.mode-nav-dropdown-btn');
+                    if (btn) btn.setAttribute('aria-expanded', 'true');
+                }
+            });
+        }
+
+        function saveNavGroupState() {
+            const NAV_STATE_KEY = 'intercept_nav_groups';
+            const state = {};
+            document.querySelectorAll('.mode-nav-dropdown[data-group]').forEach(dropdown => {
+                state[dropdown.dataset.group] = dropdown.classList.contains('open');
+            });
+            try {
+                localStorage.setItem(NAV_STATE_KEY, JSON.stringify(state));
+            } catch (e) { /* storage full or unavailable */ }
         }
 
         function closeAllDropdowns() {
diff --git a/tests/test_nav_state.py b/tests/test_nav_state.py
new file mode 100644
index 0000000..d8a858d
--- /dev/null
+++ b/tests/test_nav_state.py
@@ -0,0 +1,25 @@
+"""Tests for nav group localStorage persistence (JS logic verified via structure check)."""
+
+
+def _logged_in_get(client, path):
+    """Make a GET request with a pre-seeded logged-in session."""
+    with client.session_transaction() as sess:
+        sess["logged_in"] = True
+    return client.get(path)
+
+
+def test_index_page_includes_nav_state_init(client):
+    """nav group init function must be present in the index page."""
+    resp = _logged_in_get(client, "/")
+    assert resp.status_code == 200
+    html = resp.data.decode()
+    assert "initNavGroupState" in html
+    assert "localStorage" in html
+
+
+def test_nav_groups_have_data_group_attributes(client):
+    """Each nav group must have a data-group attribute for state keying."""
+    resp = _logged_in_get(client, "/")
+    html = resp.data.decode()
+    for group in ["signals", "tracking", "space", "wireless", "intel"]:
+        assert f'data-group="{group}"' in html, f"Missing data-group={group}"