diff --git a/modules/markup/html.go b/modules/markup/html.go
index 1c2ae6918d..c2339838e5 100644
--- a/modules/markup/html.go
+++ b/modules/markup/html.go
@@ -254,26 +254,35 @@ func RenderTocHeadingItems(ctx *RenderContext, nodeDetailsAttrs map[string]strin
 	currentLevel := baseLevel
 	indent := []byte{' ', ' '}
 	_, _ = htmlutil.HTMLPrint(out, "<ul>\n")
-	for _, header := range ctx.TocHeadingItems {
+	for i, header := range ctx.TocHeadingItems {
+		// Go deeper: open nested <ul> elements (wrapped in <li> for valid HTML)
 		for currentLevel < header.HeadingLevel {
 			_, _ = out.Write(indent)
-			_, _ = htmlutil.HTMLPrint(out, "<ul>\n")
+			_, _ = htmlutil.HTMLPrint(out, "<li><ul>\n")
 			indent = append(indent, ' ', ' ')
 			currentLevel++
 		}
+		// Go shallower: close nested </ul></li> elements
 		for currentLevel > header.HeadingLevel {
 			indent = indent[:len(indent)-2]
 			_, _ = out.Write(indent)
-			_, _ = htmlutil.HTMLPrint(out, "</ul>\n")
+			_, _ = htmlutil.HTMLPrint(out, "</ul></li>\n")
 			currentLevel--
 		}
 		_, _ = out.Write(indent)
-		_, _ = htmlutil.HTMLPrintf(out, "<li><a href=\"#%s\">%s</a></li>\n", header.AnchorID, header.InnerText)
+		_, _ = htmlutil.HTMLPrintf(out, "<li><a href=\"#%s\">%s</a>", header.AnchorID, header.InnerText)
+		// Check if next item is at a deeper level - if so, don't close the <li> yet
+		nextIsDeeper := i+1 < len(ctx.TocHeadingItems) && ctx.TocHeadingItems[i+1].HeadingLevel > header.HeadingLevel
+		if !nextIsDeeper {
+			_, _ = htmlutil.HTMLPrint(out, "</li>")
+		}
+		_, _ = htmlutil.HTMLPrint(out, "\n")
 	}
+	// Close any remaining nested levels
 	for currentLevel > baseLevel {
 		indent = indent[:len(indent)-2]
 		_, _ = out.Write(indent)
-		_, _ = htmlutil.HTMLPrint(out, "</ul>\n")
+		_, _ = htmlutil.HTMLPrint(out, "</ul></li>\n")
 		currentLevel--
 	}
 	_, _ = htmlutil.HTMLPrint(out, "</ul>\n</details>\n")
diff --git a/modules/markup/html_toc_test.go b/modules/markup/html_toc_test.go
index e93cfc9346..ab363878a4 100644
--- a/modules/markup/html_toc_test.go
+++ b/modules/markup/html_toc_test.go
@@ -38,16 +38,16 @@ include_toc: true
 	result = re.ReplaceAllString(result, "\n")
 	expected := `<details><summary>toc</summary>
 <ul>
-  <li><a href="#user-content-tag-link-and-bold" rel="nofollow">tag link and Bold</a></li>
-  <ul>
-    <li><a href="#user-content-code-block-a" rel="nofollow">code block &lt;a&gt;</a></li>
-    <ul>
-      <ul>
+  <li><a href="#user-content-tag-link-and-bold" rel="nofollow">tag link and Bold</a>
+  <li><ul>
+    <li><a href="#user-content-code-block-a" rel="nofollow">code block &lt;a&gt;</a>
+    <li><ul>
+      <li><ul>
         <li><a href="#user-content-markdown-bold" rel="nofollow">markdown bold</a></li>
-      </ul>
-    </ul>
+      </ul></li>
+    </ul></li>
     <li><a href="#user-content-last" rel="nofollow">last</a></li>
-  </ul>
+  </ul></li>
 </ul>
 </details>