diff --git a/docs/extensions/index.md b/docs/extensions/index.md
index a9d58b3..0edbe05 100644
--- a/docs/extensions/index.md
+++ b/docs/extensions/index.md
@@ -1021,9 +1021,26 @@ $tocExtension = new TableOfContentsExtension(
     cssClass: 'toc',   // CSS class for nav element
     position: 'top',   // 'top', 'bottom', or null for manual placement
     separator: '<hr>', // Optional HTML between TOC and content
+    collapsible: true, // Wrap in a <details>/<summary> disclosure
+    summary: 'Contents', // Disclosure label (default 'Table of Contents')
+    open: false,       // Start expanded when true (default collapsed)
 );
 ```
 
+**Collapsible:** with `collapsible: true` the TOC is wrapped in a
+`<details>`/`<summary>` disclosure (closed unless `open: true`), with the heading
+list directly inside it:
+
+```html
+<details class="toc">
+<summary>Table of Contents</summary>
+<ul> ... </ul>
+</details>
+```
+
+When `collapsible` is off (the default) the output is the unchanged
+`<nav class="toc">`.
+
 **Auto-insertion:**
 
 ```php
diff --git a/src/Extension/TableOfContentsExtension.php b/src/Extension/TableOfContentsExtension.php
index 5fb6de2..2fffbec 100644
--- a/src/Extension/TableOfContentsExtension.php
+++ b/src/Extension/TableOfContentsExtension.php
@@ -39,6 +39,9 @@
  *     listType: 'ol', // Use ordered list
  *     position: 'top', // Auto-insert at 'top', 'bottom', or null for manual
  *     separator: "<hr>\n", // Optional separator between TOC and content
+ *     collapsible: true, // Wrap in a <details>/<summary> disclosure
+ *     summary: 'Contents', // Disclosure label (default 'Table of Contents')
+ *     open: false, // Start expanded when true (default collapsed)
  * );
  * ```
  */
@@ -58,6 +61,10 @@ class TableOfContentsExtension implements ResettableExtensionInterface
      * @param string $cssClass CSS class for the TOC container
      * @param string|null $position Auto-insert position: 'top', 'bottom', or null for manual placement
      * @param string $separator HTML separator between TOC and content (when position is set)
+     * @param bool $collapsible Wrap the TOC in a `<details>`/`<summary>` disclosure so it can be
+     *   collapsed. Off by default; when off the output is the unchanged `<nav class="toc">`.
+     * @param string $summary Summary label for the disclosure (only used when $collapsible is true).
+     * @param bool $open Render the disclosure expanded by default (only used when $collapsible is true).
      */
     public function __construct(
         protected int $minLevel = 1,
@@ -66,6 +73,9 @@ public function __construct(
         protected string $cssClass = 'toc',
         protected ?string $position = null,
         protected string $separator = '',
+        protected bool $collapsible = false,
+        protected string $summary = 'Table of Contents',
+        protected bool $open = false,
     ) {
     }
 
@@ -169,9 +179,19 @@ protected function renderTocHtml(array $headings): string
             return '';
         }
 
-        $html = '<nav class="' . StringUtil::escapeHtml($this->cssClass) . '">' . "\n";
+        if (!$this->collapsible) {
+            return '<nav class="' . StringUtil::escapeHtml($this->cssClass) . '">' . "\n"
+                . $this->renderTocList($headings)
+                . '</nav>' . "\n";
+        }
+
+        // Collapsible: the heading list sits directly inside a <details>
+        // disclosure so it can be toggled, closed by default unless $open.
+        $open = $this->open ? ' open' : '';
+        $html = '<details class="' . StringUtil::escapeHtml($this->cssClass) . '"' . $open . '>' . "\n";
+        $html .= '<summary>' . StringUtil::escapeHtml($this->summary) . '</summary>' . "\n";
         $html .= $this->renderTocList($headings);
-        $html .= '</nav>' . "\n";
+        $html .= '</details>' . "\n";
 
         return $html;
     }
diff --git a/tests/TestCase/Extension/TableOfContentsExtensionTest.php b/tests/TestCase/Extension/TableOfContentsExtensionTest.php
index 385eee8..098c36f 100644
--- a/tests/TestCase/Extension/TableOfContentsExtensionTest.php
+++ b/tests/TestCase/Extension/TableOfContentsExtensionTest.php
@@ -430,4 +430,64 @@ public function testSkippedHeadingLevelsFollowedByShallowerHeadingReuseSameNeste
         $this->assertStringNotContainsString("</ul>\n\n<ul>", $html);
         $this->assertStringContainsString('<li><a href="#Three">Three</a></li>' . "\n<li><a href=\"#Two\">Two</a></li>", $html);
     }
+
+    public function testNonCollapsibleTocKeepsPlainNav(): void
+    {
+        $converter = new DjotConverter();
+        $tocExtension = new TableOfContentsExtension();
+        $converter->addExtension($tocExtension);
+
+        $converter->convert("# One\n\n## Two\n");
+        $html = $tocExtension->getTocHtml();
+
+        $this->assertStringStartsWith('<nav class="toc">', $html);
+        $this->assertStringNotContainsString('<details', $html);
+    }
+
+    public function testCollapsibleWrapsTocInClosedDisclosure(): void
+    {
+        $converter = new DjotConverter();
+        $tocExtension = new TableOfContentsExtension(collapsible: true);
+        $converter->addExtension($tocExtension);
+
+        $converter->convert("# One\n\n## Two\n");
+        $html = $tocExtension->getTocHtml();
+
+        // Closed by default (no `open`), list sits directly inside <details>.
+        $this->assertStringStartsWith(
+            '<details class="toc">' . "\n" . '<summary>Table of Contents</summary>' . "\n" . '<ul>',
+            $html,
+        );
+        $this->assertStringEndsWith('</ul>' . "\n" . '</details>' . "\n", $html);
+        $this->assertStringNotContainsString('<nav', $html);
+        $this->assertStringNotContainsString('<details class="toc" open', $html);
+    }
+
+    public function testCollapsibleHonorsOpenAndCustomSummary(): void
+    {
+        $converter = new DjotConverter();
+        $tocExtension = new TableOfContentsExtension(collapsible: true, summary: 'Contents', open: true);
+        $converter->addExtension($tocExtension);
+
+        $converter->convert("# One\n");
+        $html = $tocExtension->getTocHtml();
+
+        $this->assertStringStartsWith(
+            '<details class="toc" open>' . "\n" . '<summary>Contents</summary>',
+            $html,
+        );
+    }
+
+    public function testCollapsibleSummaryIsHtmlEscaped(): void
+    {
+        $converter = new DjotConverter();
+        $tocExtension = new TableOfContentsExtension(collapsible: true, summary: 'A & <b>B</b>');
+        $converter->addExtension($tocExtension);
+
+        $converter->convert("# One\n");
+        $html = $tocExtension->getTocHtml();
+
+        $this->assertStringContainsString('<summary>A &amp; &lt;b&gt;B&lt;/b&gt;</summary>', $html);
+        $this->assertStringNotContainsString('<b>B</b>', $html);
+    }
 }