diff --git a/muxplex/frontend/app.js b/muxplex/frontend/app.js
index 9f01d2f..94a1d26 100644
--- a/muxplex/frontend/app.js
+++ b/muxplex/frontend/app.js
@@ -1409,12 +1409,13 @@ async function patchServerSetting(key, value) {
 }
 
 /**
- * Build a single remote instance row element with URL input, name input, and remove button.
+ * Build a single remote instance row element with URL input, name input, key input, and remove button.
  * @param {string} url - remote instance URL
  * @param {string} name - remote instance display name
+ * @param {string} key - federation key for the remote instance
  * @returns {HTMLDivElement}
  */
-function _buildRemoteInstanceRow(url, name) {
+function _buildRemoteInstanceRow(url, name, key) {
   var row = document.createElement('div');
   row.className = 'settings-remote-row';
   var urlInput = document.createElement('input');
@@ -1429,12 +1430,19 @@ function _buildRemoteInstanceRow(url, name) {
   nameInput.placeholder = 'Device name';
   nameInput.value = name || '';
   nameInput.setAttribute('aria-label', 'Remote instance display name');
+  var keyInput = document.createElement('input');
+  keyInput.type = 'password';
+  keyInput.className = 'settings-remote-key';
+  keyInput.placeholder = 'Federation key';
+  keyInput.value = key || '';
+  keyInput.setAttribute('aria-label', 'Federation key for remote instance');
   var removeBtn = document.createElement('button');
   removeBtn.className = 'settings-remote-remove';
   removeBtn.textContent = '\u00d7';
   removeBtn.setAttribute('aria-label', 'Remove remote instance');
   row.appendChild(urlInput);
   row.appendChild(nameInput);
+  row.appendChild(keyInput);
   row.appendChild(removeBtn);
   return row;
 }
@@ -1449,10 +1457,12 @@ function _saveRemoteInstances() {
   container.querySelectorAll('.settings-remote-row').forEach(function(row) {
     var urlEl = row.querySelector('.settings-remote-url');
     var nameEl = row.querySelector('.settings-remote-name');
+    var keyEl = row.querySelector('.settings-remote-key');
     var url = (urlEl && urlEl.value) ? urlEl.value.trim() : '';
     var name = (nameEl && nameEl.value) ? nameEl.value.trim() : '';
+    var key = (keyEl && keyEl.value) ? keyEl.value.trim() : '';
     if (url) {
-      instances.push({ url: url, name: name });
+      instances.push({ url: url, name: name, key: key });
     }
   });
   patchServerSetting('remote_instances', instances);
@@ -1799,7 +1809,7 @@ function openSettings() {
       remoteInstancesEl.innerHTML = '';
       var remotes = (ss && ss.remote_instances) || [];
       remotes.forEach(function(r) {
-        remoteInstancesEl.appendChild(_buildRemoteInstanceRow(r.url || '', r.name || ''));
+        remoteInstancesEl.appendChild(_buildRemoteInstanceRow(r.url || '', r.name || '', r.key || ''));
       });
     }
 
@@ -2336,11 +2346,25 @@ function bindStaticEventListeners() {
     }, 500);
   });
 
+  // Multi-Device tab — federation generate key button
+  on($('federation-generate-btn'), 'click', function() {
+    api('POST', '/api/federation/generate-key').then(function(data) {
+      var displayEl = $('federation-key-display');
+      if (displayEl && data && data.key) {
+        displayEl.textContent = data.key;
+        displayEl.classList.add('settings-key-display--visible');
+      }
+      showToast('Federation key generated');
+    }).catch(function() {
+      showToast('Failed to generate federation key');
+    });
+  });
+
   // Multi-Device tab — add remote instance button
   on($('add-remote-instance-btn'), 'click', function() {
     var container = $('setting-remote-instances');
     if (container) {
-      container.appendChild(_buildRemoteInstanceRow('', ''));
+      container.appendChild(_buildRemoteInstanceRow('', '', ''));
     }
   });
 
diff --git a/muxplex/frontend/index.html b/muxplex/frontend/index.html
index cee7282..407cf90 100644
--- a/muxplex/frontend/index.html
+++ b/muxplex/frontend/index.html
@@ -242,6 +242,14 @@
                 <option value="local">Local Only</option>
               </select>
             </div>
+            <div class="settings-field settings-field--column">
+              <label class="settings-label">Federation Key</label>
+              <div class="settings-federation-key">
+                <span id="federation-key-display" class="settings-key-display">&#8226;&#8226;&#8226;&#8226;&#8226;&#8226;&#8226;&#8226;</span>
+                <button id="federation-generate-btn" class="settings-action-btn">Generate Key</button>
+              </div>
+              <span class="settings-helper">Share this key with remote instances to allow federation access</span>
+            </div>
             <div class="settings-field settings-field--column">
               <label class="settings-label">Remote Instances</label>
               <div id="setting-remote-instances" class="settings-remote-list"></div>
diff --git a/muxplex/frontend/style.css b/muxplex/frontend/style.css
index dca90e7..1274cb4 100644
--- a/muxplex/frontend/style.css
+++ b/muxplex/frontend/style.css
@@ -1746,3 +1746,36 @@ body {
 #setting-device-name {
   max-width: 200px;
 }
+
+/* Federation key display */
+.settings-federation-key {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+}
+
+.settings-key-display {
+  font-family: var(--font-mono);
+  font-size: 13px;
+  color: var(--text-muted);
+  user-select: all;
+}
+
+.settings-key-display--visible {
+  color: var(--accent);
+  background: var(--accent-dim);
+  padding: 4px 8px;
+  border-radius: 4px;
+}
+
+/* Remote instance federation key input */
+.settings-remote-key {
+  flex: 1;
+  min-width: 100px;
+  font-size: 12px;
+  padding: 4px 6px;
+  background: var(--bg);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  color: var(--text);
+}
diff --git a/muxplex/tests/test_frontend_html.py b/muxplex/tests/test_frontend_html.py
index 3377f47..67ec556 100644
--- a/muxplex/tests/test_frontend_html.py
+++ b/muxplex/tests/test_frontend_html.py
@@ -1337,3 +1337,37 @@ def test_html_display_panel_no_view_scope() -> None:
     assert el is None, (
         "#setting-view-scope must NOT be in the display panel (moved to Multi-Device tab)"
     )
+
+
+# ============================================================
+# Federation Key Management UI (task-6-federation-key-ui)
+# ============================================================
+
+
+def test_html_settings_federation_key_section() -> None:
+    """Multi-Device tab must contain federation-key-display and federation-generate-btn."""
+    soup = _SOUP
+    devices_panel = soup.find("div", attrs={"data-tab": "devices"})
+    assert devices_panel is not None, "Missing devices panel (data-tab='devices')"
+    key_display = devices_panel.find(id="federation-key-display")
+    assert key_display is not None, (
+        "Missing #federation-key-display inside devices panel"
+    )
+    generate_btn = devices_panel.find(id="federation-generate-btn")
+    assert generate_btn is not None, (
+        "Missing #federation-generate-btn inside devices panel"
+    )
+    assert generate_btn.name == "button", (
+        f"#federation-generate-btn must be a <button>, got: {generate_btn.name}"
+    )
+
+
+def test_html_settings_remote_instance_key_input() -> None:
+    """Multi-Device tab must contain #setting-remote-instances container."""
+    soup = _SOUP
+    devices_panel = soup.find("div", attrs={"data-tab": "devices"})
+    assert devices_panel is not None, "Missing devices panel (data-tab='devices')"
+    el = devices_panel.find(id="setting-remote-instances")
+    assert el is not None, (
+        "Missing #setting-remote-instances inside devices panel"
+    )