diff --git a/static/css/styles.css b/static/css/styles.css
index 47f2288..99e65b5 100644
--- a/static/css/styles.css
+++ b/static/css/styles.css
@@ -1677,6 +1677,45 @@ pre code {
   font-size: var(--text-md);
 }
 
+.setup-complete-btn {
+  min-width: 12.5rem;
+}
+
+.setup-complete-ready,
+.setup-complete-pending {
+  align-items: center;
+  gap: var(--sp-2);
+}
+
+.setup-complete-ready {
+  display: inline-flex;
+}
+
+.setup-complete-pending {
+  display: none;
+}
+
+.setup-complete-btn.is-async-pending .setup-complete-ready {
+  display: none;
+}
+
+.setup-complete-btn.is-async-pending .setup-complete-pending {
+  display: inline-flex;
+}
+
+.setup-complete-spinner {
+  width: 1rem;
+  height: 1rem;
+  border: 2px solid hsla(0, 0%, 100%, 0.35);
+  border-top-color: currentColor;
+  border-radius: 50%;
+  animation: setup-complete-spin 0.8s linear infinite;
+}
+
+@keyframes setup-complete-spin {
+  to { transform: rotate(360deg); }
+}
+
 .setup-nav-note {
   margin-top: var(--sp-4);
   display: inline-flex;
diff --git a/templates/setup.html b/templates/setup.html
index a990a16..9ccb249 100644
--- a/templates/setup.html
+++ b/templates/setup.html
@@ -349,8 +349,12 @@ <h3>Schedule & Bandwidth</h3>
                                 <div class="invalid-feedback">Bandwidth limit must be a valid format (e.g. 4M, 1G, 500k).</div>
                             </div>
                             <div class="setup-actions">
-                                <button onclick="saveSchedule()" class="btn btn-primary btn-lg">
-                                    <i class="fas fa-check me-1"></i> Complete Setup
+                                <button id="setupCompleteBtn" type="button" onclick="saveSchedule()" class="btn btn-primary btn-lg setup-complete-btn">
+                                    <span class="setup-complete-ready"><i class="fas fa-check me-1"></i> Complete Setup</span>
+                                    <span class="setup-complete-pending">
+                                        <span class="setup-complete-spinner"></span>
+                                        Completing Setup
+                                    </span>
                                 </button>
                             </div>
                         </div>
@@ -412,6 +416,7 @@ <h5 id="confirmationModalTitle">Confirm Action</h5>
     <script>
         let currentStep = 1;
         const totalSteps = 6;
+        let setupCompletionPending = false;
 
         // --- LocalStorage step persistence ---
         function saveStepToStorage(step) {
@@ -721,21 +726,28 @@ <h5 id="confirmationModalTitle">Confirm Action</h5>
 
         // --- Schedule ---
         async function saveSchedule() {
+            if (setupCompletionPending) return;
             hideError();
             const valid = [validateBackupTime(), validateBandwidthLimit()];
             if (valid.includes(false)) { showError('Please correct the highlighted fields.'); return; }
             const time = document.getElementById('backupTime').value;
             const bandwidthLimit = document.getElementById('bandwidthLimit').value;
             if (!time) { showError('Please select a backup time.'); return; }
-            const saveBtn = document.querySelector('button[onclick="saveSchedule()"]');
+            const saveBtn = document.getElementById('setupCompleteBtn');
+            setupCompletionPending = true;
             window.AsyncButtonState.start(saveBtn);
             try {
                 await window.ApiClient.fetchJson('/api/setup/schedule', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ time, bandwidth_limit: bandwidthLimit }) });
                 hideError();
                 const completed = await completeSetup();
                 if (completed) window.AsyncButtonState.success(saveBtn);
-                else window.AsyncButtonState.error(saveBtn);
-            } catch (error) { window.AsyncButtonState.error(saveBtn); showError('Error saving schedule: ' + (error.message || error)); }
+                // Successful completion redirects away, so only clear this lock when the page stays here.
+                else { setupCompletionPending = false; window.AsyncButtonState.error(saveBtn); }
+            } catch (error) {
+                setupCompletionPending = false;
+                window.AsyncButtonState.error(saveBtn);
+                showError('Error saving schedule: ' + (error.message || error));
+            }
         }
 
         // --- Complete ---
diff --git a/tests/test_app_factory_routes.py b/tests/test_app_factory_routes.py
index b8b568e..b27b09c 100644
--- a/tests/test_app_factory_routes.py
+++ b/tests/test_app_factory_routes.py
@@ -225,6 +225,27 @@ def test_setup_title_keeps_product_name_before_server_name_is_chosen():
         runtime._fake_state = previous_fake_state
 
 
+def test_setup_complete_button_renders_busy_state_markup():
+    previous_runtime = runtime._runtime
+    previous_fake_state = runtime._fake_state
+    try:
+        with TemporaryDirectory() as temp_dir:
+            app = _create_fake_app(temp_dir)
+
+            with app.test_client() as client:
+                response = client.get("/setup")
+
+            assert response.status_code == 200
+            page = response.get_data(as_text=True)
+            assert 'id="setupCompleteBtn"' in page
+            assert "setup-complete-spinner" in page
+            assert "Completing Setup" in page
+            assert "setupCompletionPending" in page
+    finally:
+        runtime._runtime = previous_runtime
+        runtime._fake_state = previous_fake_state
+
+
 def test_smb_share_list_returns_validation_error_for_malformed_sss_shares_file():
     previous_runtime = runtime._runtime
     previous_fake_state = runtime._fake_state