diff --git a/app/web/templates/webhooks/form.html b/app/web/templates/webhooks/form.html
index 5595055..df350c9 100644
--- a/app/web/templates/webhooks/form.html
+++ b/app/web/templates/webhooks/form.html
@@ -123,6 +123,58 @@
 
                     <hr class="my-4">
 
+                    <!-- Webhook Template -->
+                    <h5 class="card-title mb-3">Webhook Template</h5>
+                    <div class="alert alert-info small">
+                        <i class="bi bi-info-circle"></i>
+                        Customize the webhook payload using Jinja2 templates. Leave empty to use the default JSON format.
+                    </div>
+
+                    <div class="mb-3">
+                        <label for="preset_selector" class="form-label">Load Preset Template</label>
+                        <select class="form-select" id="preset_selector">
+                            <option value="">-- Select a preset --</option>
+                        </select>
+                        <div class="form-text">Choose from pre-built templates for popular services</div>
+                    </div>
+
+                    <div class="mb-3">
+                        <label for="template_format" class="form-label">Template Format</label>
+                        <select class="form-select" id="template_format" name="template_format">
+                            <option value="json">JSON</option>
+                            <option value="text">Plain Text</option>
+                        </select>
+                        <div class="form-text">Output format of the rendered template</div>
+                    </div>
+
+                    <div class="mb-3">
+                        <label for="template" class="form-label">Template</label>
+                        <textarea class="form-control font-monospace" id="template" name="template" rows="12"
+                            placeholder="Leave empty for default format, or enter custom Jinja2 template..."></textarea>
+                        <div class="form-text">
+                            Available variables: <code>{{ "{{" }} alert.* {{ "}}" }}</code>, <code>{{ "{{" }} scan.* {{ "}}" }}</code>, <code>{{ "{{" }} rule.* {{ "}}" }}</code>
+                            <a href="#" data-bs-toggle="modal" data-bs-target="#variablesModal">View all variables</a>
+                        </div>
+                    </div>
+
+                    <div class="mb-3">
+                        <label for="content_type_override" class="form-label">Custom Content-Type (optional)</label>
+                        <input type="text" class="form-control font-monospace" id="content_type_override" name="content_type_override"
+                            placeholder="e.g., application/json, text/plain, text/markdown">
+                        <div class="form-text">Override the default Content-Type header (auto-detected from template format if not set)</div>
+                    </div>
+
+                    <div class="mb-3">
+                        <button type="button" class="btn btn-outline-secondary btn-sm" id="preview-template-btn">
+                            <i class="bi bi-eye"></i> Preview Template
+                        </button>
+                        <button type="button" class="btn btn-outline-secondary btn-sm ms-2" id="clear-template-btn">
+                            <i class="bi bi-x-circle"></i> Clear Template
+                        </button>
+                    </div>
+
+                    <hr class="my-4">
+
                     <!-- Advanced Settings -->
                     <h5 class="card-title mb-3">Advanced Settings</h5>
 
@@ -166,7 +218,7 @@
                 <h5 class="card-title"><i class="bi bi-info-circle"></i> Help</h5>
 
                 <h6 class="mt-3">Payload Format</h6>
-                <p class="small text-muted">Webhooks receive JSON payloads with alert details:</p>
+                <p class="small text-muted">Default JSON payload format (can be customized with templates):</p>
                 <pre class="small bg-dark text-light p-2 rounded"><code>{
   "event": "alert.created",
   "alert": {
@@ -181,6 +233,9 @@
   "rule": {...}
 }</code></pre>
 
+                <h6 class="mt-3">Custom Templates</h6>
+                <p class="small text-muted">Use Jinja2 templates to customize payloads for services like Slack, Discord, Gotify, or create your own format. Select a preset or write a custom template.</p>
+
                 <h6 class="mt-3">Authentication Types</h6>
                 <ul class="small">
                     <li><strong>None:</strong> No authentication</li>
@@ -196,6 +251,92 @@
     </div>
 </div>
 
+<!-- Template Variables Modal -->
+<div class="modal fade" id="variablesModal" tabindex="-1">
+    <div class="modal-dialog modal-lg">
+        <div class="modal-content">
+            <div class="modal-header">
+                <h5 class="modal-title">Available Template Variables</h5>
+                <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
+            </div>
+            <div class="modal-body">
+                <h6>Alert Variables</h6>
+                <ul class="small font-monospace">
+                    <li>{{ "{{" }} alert.id {{ "}}" }} - Alert ID</li>
+                    <li>{{ "{{" }} alert.type {{ "}}" }} - Alert type (unexpected_port, cert_expiry, etc.)</li>
+                    <li>{{ "{{" }} alert.severity {{ "}}" }} - Severity level (critical, warning, info)</li>
+                    <li>{{ "{{" }} alert.message {{ "}}" }} - Human-readable alert message</li>
+                    <li>{{ "{{" }} alert.ip_address {{ "}}" }} - IP address (if applicable)</li>
+                    <li>{{ "{{" }} alert.port {{ "}}" }} - Port number (if applicable)</li>
+                    <li>{{ "{{" }} alert.acknowledged {{ "}}" }} - Boolean: is acknowledged</li>
+                    <li>{{ "{{" }} alert.created_at {{ "}}" }} - Alert creation timestamp</li>
+                </ul>
+
+                <h6 class="mt-3">Scan Variables</h6>
+                <ul class="small font-monospace">
+                    <li>{{ "{{" }} scan.id {{ "}}" }} - Scan ID</li>
+                    <li>{{ "{{" }} scan.title {{ "}}" }} - Scan title from config</li>
+                    <li>{{ "{{" }} scan.timestamp {{ "}}" }} - Scan start time</li>
+                    <li>{{ "{{" }} scan.duration {{ "}}" }} - Scan duration in seconds</li>
+                    <li>{{ "{{" }} scan.status {{ "}}" }} - Scan status (running, completed, failed)</li>
+                    <li>{{ "{{" }} scan.triggered_by {{ "}}" }} - How scan was triggered (manual, scheduled, api)</li>
+                </ul>
+
+                <h6 class="mt-3">Rule Variables</h6>
+                <ul class="small font-monospace">
+                    <li>{{ "{{" }} rule.id {{ "}}" }} - Rule ID</li>
+                    <li>{{ "{{" }} rule.name {{ "}}" }} - Rule name</li>
+                    <li>{{ "{{" }} rule.type {{ "}}" }} - Rule type</li>
+                    <li>{{ "{{" }} rule.threshold {{ "}}" }} - Rule threshold value</li>
+                    <li>{{ "{{" }} rule.severity {{ "}}" }} - Rule severity</li>
+                </ul>
+
+                <h6 class="mt-3">App Variables</h6>
+                <ul class="small font-monospace">
+                    <li>{{ "{{" }} app.name {{ "}}" }} - Application name</li>
+                    <li>{{ "{{" }} app.version {{ "}}" }} - Application version</li>
+                    <li>{{ "{{" }} app.url {{ "}}" }} - Repository URL</li>
+                </ul>
+
+                <h6 class="mt-3">Other Variables</h6>
+                <ul class="small font-monospace">
+                    <li>{{ "{{" }} timestamp {{ "}}" }} - Current UTC timestamp</li>
+                </ul>
+
+                <h6 class="mt-3">Jinja2 Features</h6>
+                <p class="small">Templates support Jinja2 syntax including:</p>
+                <ul class="small">
+                    <li>Conditionals: <code>{{ "{%" }} if alert.severity == 'critical' {{ "%}" }}...{{ "{%" }} endif {{ "%}" }}</code></li>
+                    <li>Filters: <code>{{ "{{" }} alert.type|upper {{ "}}" }}</code>, <code>{{ "{{" }} alert.created_at.isoformat() {{ "}}" }}</code></li>
+                    <li>Default values: <code>{{ "{{" }} alert.port|default('N/A') {{ "}}" }}</code></li>
+                </ul>
+            </div>
+            <div class="modal-footer">
+                <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
+            </div>
+        </div>
+    </div>
+</div>
+
+<!-- Template Preview Modal -->
+<div class="modal fade" id="previewModal" tabindex="-1">
+    <div class="modal-dialog modal-lg">
+        <div class="modal-content">
+            <div class="modal-header">
+                <h5 class="modal-title">Template Preview</h5>
+                <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
+            </div>
+            <div class="modal-body">
+                <p class="small text-muted">Preview using sample data:</p>
+                <pre class="bg-dark text-light p-3 rounded" id="preview-output" style="max-height: 500px; overflow-y: auto;"><code></code></pre>
+            </div>
+            <div class="modal-footer">
+                <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
+            </div>
+        </div>
+    </div>
+</div>
+
 {% endblock %}
 
 {% block scripts %}
@@ -203,6 +344,106 @@
 const webhookId = {{ webhook.id if webhook else 'null' }};
 const mode = '{{ mode }}';
 
+// Load template presets on page load
+async function loadPresets() {
+    try {
+        const response = await fetch('/api/webhooks/template-presets');
+        const data = await response.json();
+
+        if (data.status === 'success') {
+            const selector = document.getElementById('preset_selector');
+            data.presets.forEach(preset => {
+                const option = document.createElement('option');
+                option.value = JSON.stringify({
+                    template: preset.template,
+                    format: preset.format,
+                    content_type: preset.content_type
+                });
+                option.textContent = `${preset.name} - ${preset.description}`;
+                selector.appendChild(option);
+            });
+        }
+    } catch (error) {
+        console.error('Failed to load presets:', error);
+    }
+}
+
+// Handle preset selection
+document.getElementById('preset_selector').addEventListener('change', function() {
+    if (!this.value) return;
+
+    try {
+        const preset = JSON.parse(this.value);
+        document.getElementById('template').value = preset.template;
+        document.getElementById('template_format').value = preset.format;
+        document.getElementById('content_type_override').value = preset.content_type;
+    } catch (error) {
+        console.error('Failed to load preset:', error);
+    }
+});
+
+// Handle preview template button
+document.getElementById('preview-template-btn').addEventListener('click', async function() {
+    const template = document.getElementById('template').value.trim();
+    if (!template) {
+        alert('Please enter a template first');
+        return;
+    }
+
+    const templateFormat = document.getElementById('template_format').value;
+
+    try {
+        const response = await fetch('/api/webhooks/preview-template', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify({
+                template: template,
+                template_format: templateFormat
+            })
+        });
+
+        const result = await response.json();
+
+        if (result.status === 'success') {
+            // Display preview in modal
+            const output = document.querySelector('#preview-output code');
+            if (templateFormat === 'json') {
+                // Pretty print JSON
+                try {
+                    const parsed = JSON.parse(result.rendered);
+                    output.textContent = JSON.stringify(parsed, null, 2);
+                } catch (e) {
+                    output.textContent = result.rendered;
+                }
+            } else {
+                output.textContent = result.rendered;
+            }
+
+            // Show modal
+            const modal = new bootstrap.Modal(document.getElementById('previewModal'));
+            modal.show();
+        } else {
+            alert(`Preview failed: ${result.message}`);
+        }
+    } catch (error) {
+        console.error('Error previewing template:', error);
+        alert('Failed to preview template');
+    }
+});
+
+// Handle clear template button
+document.getElementById('clear-template-btn').addEventListener('click', function() {
+    if (confirm('Clear template and reset to default format?')) {
+        document.getElementById('template').value = '';
+        document.getElementById('template_format').value = 'json';
+        document.getElementById('content_type_override').value = '';
+        document.getElementById('preset_selector').value = '';
+    }
+});
+
+// Load presets on page load
+loadPresets();
+
 // Show/hide auth fields based on type
 document.getElementById('auth_type').addEventListener('change', function() {
     const authType = this.value;
@@ -268,6 +509,17 @@ async function loadWebhookData(id) {
                 if (checkbox) checkbox.checked = true;
             });
         }
+
+        // Load template fields
+        if (webhook.template) {
+            document.getElementById('template').value = webhook.template;
+        }
+        if (webhook.template_format) {
+            document.getElementById('template_format').value = webhook.template_format;
+        }
+        if (webhook.content_type_override) {
+            document.getElementById('content_type_override').value = webhook.content_type_override;
+        }
     } catch (error) {
         console.error('Error loading webhook:', error);
         alert('Failed to load webhook data');
@@ -318,6 +570,18 @@ document.getElementById('webhook-form').addEventListener('submit', async functio
         formData.severity_filter = severities;
     }
 
+    // Add template fields
+    const template = document.getElementById('template').value.trim();
+    if (template) {
+        formData.template = template;
+        formData.template_format = document.getElementById('template_format').value;
+
+        const contentTypeOverride = document.getElementById('content_type_override').value.trim();
+        if (contentTypeOverride) {
+            formData.content_type_override = contentTypeOverride;
+        }
+    }
+
     try {
         const url = mode === 'edit' ? `/api/webhooks/${webhookId}` : '/api/webhooks';
         const method = mode === 'edit' ? 'PUT' : 'POST';
diff --git a/docs/API_REFERENCE.md b/docs/API_REFERENCE.md
index e8c519b..47e73e9 100644
--- a/docs/API_REFERENCE.md
+++ b/docs/API_REFERENCE.md
@@ -2195,6 +2195,9 @@ Create a new webhook configuration.
 | `severity_filter` | array | No | Array of severities to filter (empty = all severities) |
 | `timeout` | integer | No | Request timeout in seconds (default: 10) |
 | `retry_count` | integer | No | Number of retry attempts (default: 3) |
+| `template` | string | No | Jinja2 template for custom payload (leave empty for default format) |
+| `template_format` | string | No | Template output format: json, text (default: json) |
+| `content_type_override` | string | No | Custom Content-Type header (auto-detected if not specified) |
 
 **Success Response (201 Created):**
 ```json
@@ -2258,6 +2261,19 @@ curl -X POST http://localhost:5000/api/webhooks \
     "auth_token": "your-secret-token"
   }' \
   -b cookies.txt
+
+# Create webhook with custom template
+curl -X POST http://localhost:5000/api/webhooks \
+  -H "Content-Type: application/json" \
+  -d '{
+    "name": "Gotify Notifications",
+    "url": "https://gotify.example.com/message",
+    "template": "{\"title\": \"{{ scan.title }}\", \"message\": \"{{ alert.message }}\", \"priority\": {% if alert.severity == \"critical\" %}5{% else %}3{% endif %}}",
+    "template_format": "json",
+    "auth_type": "custom",
+    "custom_headers": {"X-Gotify-Key": "your-app-token"}
+  }' \
+  -b cookies.txt
 ```
 
 ### Update Webhook
@@ -2421,6 +2437,126 @@ curl -X POST http://localhost:5000/api/webhooks/1/test \
   -b cookies.txt
 ```
 
+### Preview Template
+
+Preview a webhook template with sample data before saving.
+
+**Endpoint:** `POST /api/webhooks/preview-template`
+
+**Authentication:** Required
+
+**Request Body:**
+```json
+{
+  "template": "{\"title\": \"{{ scan.title }}\", \"message\": \"{{ alert.message }}\"}",
+  "template_format": "json"
+}
+```
+
+**Request Body Fields:**
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| `template` | string | Yes | Jinja2 template to preview |
+| `template_format` | string | No | Output format: json or text (default: json) |
+
+**Success Response (200 OK):**
+```json
+{
+  "status": "success",
+  "rendered": "{\"title\": \"Production Infrastructure Scan\", \"message\": \"Unexpected port 8080 found open on 192.168.1.100\"}",
+  "format": "json"
+}
+```
+
+**Error Responses:**
+
+*400 Bad Request* - Template validation error:
+```json
+{
+  "status": "error",
+  "message": "Template validation error: Unexpected '}}' at line 2"
+}
+```
+
+*400 Bad Request* - Template rendering error:
+```json
+{
+  "status": "error",
+  "message": "Template rendering error: undefined variable 'invalid_var'"
+}
+```
+
+**Usage Example:**
+```bash
+curl -X POST http://localhost:5000/api/webhooks/preview-template \
+  -H "Content-Type: application/json" \
+  -d '{
+    "template": "{\"title\": \"Alert\", \"message\": \"{{ alert.message }}\"}",
+    "template_format": "json"
+  }' \
+  -b cookies.txt
+```
+
+### Get Template Presets
+
+Get list of available webhook template presets for popular services.
+
+**Endpoint:** `GET /api/webhooks/template-presets`
+
+**Authentication:** Required
+
+**Success Response (200 OK):**
+```json
+{
+  "status": "success",
+  "presets": [
+    {
+      "id": "default_json",
+      "name": "Default JSON (Current Format)",
+      "description": "Standard webhook payload format matching the current implementation",
+      "format": "json",
+      "content_type": "application/json",
+      "category": "general",
+      "template": "{\n  \"event\": \"alert.created\",\n  \"alert\": {...}\n}"
+    },
+    {
+      "id": "gotify",
+      "name": "Gotify",
+      "description": "Optimized for Gotify push notification server with markdown support",
+      "format": "json",
+      "content_type": "application/json",
+      "category": "service",
+      "template": "{\n  \"title\": \"{{ scan.title }}\",\n  \"message\": \"{{ alert.message }}\",\n  \"priority\": {% if alert.severity == 'critical' %}8{% else %}5{% endif %}\n}"
+    },
+    {
+      "id": "slack",
+      "name": "Slack",
+      "description": "Rich Block Kit format for Slack webhooks with visual formatting",
+      "format": "json",
+      "content_type": "application/json",
+      "category": "service",
+      "template": "..."
+    }
+  ]
+}
+```
+
+**Available Presets:**
+- `default_json` - Standard JSON format (backward compatible)
+- `custom_json` - Flexible custom format with title/message/priority
+- `gotify` - Gotify push notification server format
+- `ntfy` - Simple text format for Ntfy pub-sub service
+- `slack` - Slack Block Kit format with rich formatting
+- `discord` - Discord embedded message format
+- `plain_text` - Simple plain text format
+
+**Usage Example:**
+```bash
+curl -X GET http://localhost:5000/api/webhooks/template-presets \
+  -b cookies.txt
+```
+
 ### Get Webhook Delivery Logs
 
 Get delivery history for a specific webhook.
@@ -2508,7 +2644,9 @@ curl -X GET "http://localhost:5000/api/webhooks/1/logs?page=2" \
 
 ### Webhook Payload Format
 
-When alerts are triggered, webhooks receive JSON payloads in this format:
+#### Default Format
+
+When alerts are triggered, webhooks without custom templates receive JSON payloads in this default format:
 
 ```json
 {
@@ -2538,6 +2676,76 @@ When alerts are triggered, webhooks receive JSON payloads in this format:
 }
 ```
 
+#### Custom Templates
+
+Webhooks support custom Jinja2 templates to customize the payload format for different services. Templates have access to the following variables:
+
+**Available Template Variables:**
+
+| Variable | Description | Example |
+|----------|-------------|---------|
+| `{{ alert.id }}` | Alert ID | `123` |
+| `{{ alert.type }}` | Alert type | `"cert_expiry"`, `"unexpected_port"` |
+| `{{ alert.severity }}` | Severity level | `"critical"`, `"warning"`, `"info"` |
+| `{{ alert.message }}` | Human-readable message | `"Certificate expires in 15 days"` |
+| `{{ alert.ip_address }}` | IP address | `"192.168.1.10"` |
+| `{{ alert.port }}` | Port number | `443` |
+| `{{ alert.acknowledged }}` | Is acknowledged | `true`, `false` |
+| `{{ alert.created_at }}` | Creation timestamp | `datetime` object |
+| `{{ scan.id }}` | Scan ID | `42` |
+| `{{ scan.title }}` | Scan title | `"Production Network Scan"` |
+| `{{ scan.status }}` | Scan status | `"completed"`, `"running"`, `"failed"` |
+| `{{ scan.duration }}` | Scan duration (seconds) | `125.5` |
+| `{{ scan.triggered_by }}` | How scan was triggered | `"manual"`, `"scheduled"`, `"api"` |
+| `{{ rule.id }}` | Rule ID | `3` |
+| `{{ rule.name }}` | Rule name | `"Certificate Expiry Warning"` |
+| `{{ rule.type }}` | Rule type | `"cert_expiry"` |
+| `{{ rule.threshold }}` | Rule threshold | `30` |
+| `{{ app.name }}` | Application name | `"SneakyScanner"` |
+| `{{ app.version }}` | Application version | `"1.0.0-phase5"` |
+| `{{ app.url }}` | Repository URL | `"https://github..."` |
+| `{{ timestamp }}` | Current UTC timestamp | `datetime` object |
+
+**Jinja2 Features:**
+- Conditionals: `{% if alert.severity == 'critical' %}...{% endif %}`
+- Filters: `{{ alert.type|upper }}`, `{{ alert.created_at.isoformat() }}`
+- Default values: `{{ alert.port|default('N/A') }}`
+
+**Template Examples:**
+
+*Custom JSON with priority:*
+```jinja2
+{
+  "title": "{{ scan.title }}",
+  "message": "{{ alert.message }}",
+  "priority": {% if alert.severity == 'critical' %}5{% elif alert.severity == 'warning' %}3{% else %}1{% endif %},
+  "alert_id": {{ alert.id }}
+}
+```
+
+*Plain text format:*
+```jinja2
+ALERT: {{ alert.severity|upper }}
+{{ alert.message }}
+
+Scan: {{ scan.title }}
+Rule: {{ rule.name }}
+Time: {{ timestamp.strftime('%Y-%m-%d %H:%M:%S UTC') }}
+```
+
+*Gotify format:*
+```jinja2
+{
+  "title": "{{ scan.title }}",
+  "message": "**{{ alert.severity|upper }}**: {{ alert.message }}",
+  "priority": {% if alert.severity == 'critical' %}8{% elif alert.severity == 'warning' %}5{% else %}2{% endif %},
+  "extras": {
+    "client::display": {"contentType": "text/markdown"},
+    "alert_id": {{ alert.id }}
+  }
+}
+```
+
 ### Authentication Types
 
 **None:**
@@ -2840,8 +3048,8 @@ API versioning will be implemented in future phases. For now, the API is conside
 - **Webhooks API** - Webhook management, delivery tracking, authentication support, retry logic
 
 ### Endpoint Count
-- Total endpoints: 65+
-- Authenticated endpoints: 60+
+- Total endpoints: 67+
+- Authenticated endpoints: 62+
 - Public endpoints: 5 (login, setup, health checks)
 
 ---
@@ -2855,5 +3063,5 @@ For issues, questions, or feature requests:
 ---
 
 **Last Updated:** 2025-11-18
-**Phase:** 5 - Alerts Management
+**Phase:** 5 - Alerts Management & Custom Webhook Templates
 **Next Update:** Phase 6 - Future Enhancements