diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index cee5b6b2e..1c104cf5e 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -51,6 +51,7 @@ jobs:
         run: |
           gem install asciidoctor -v "2.0.26"
           gem install asciidoctor-diagram -v "3.1.0"
+          gem install asciidoctor-html5s -v "0.5.1"
       - name: Install GoAT
         run: go install github.com/blampe/goat/cmd/goat@177de93b192b8ffae608e5d9ec421cc99bf68402
       - name: Install Python
diff --git a/markup/asciidocext/asciidoc_integration_test.go b/markup/asciidocext/asciidoc_integration_test.go
index 72fa54569..68a440bb6 100644
--- a/markup/asciidocext/asciidoc_integration_test.go
+++ b/markup/asciidocext/asciidoc_integration_test.go
@@ -1153,3 +1153,59 @@ func validatePublishedSite_20(b *hugolib.IntegrationTestBuilder) {
 		b.AssertFileExists(path, true)
 	}
 }
+
+// See issue 15121
+func TestAsciiDocHTML5BackendTOC(t *testing.T) {
+	if !htesting.IsRealCI() {
+		t.Skip()
+	}
+	if ok, err := asciidocext.Supports(); !ok {
+		t.Skip(err)
+	}
+
+	t.Cleanup(func() {
+		resetDefaultAsciiDocExtConfig()
+	})
+
+	files := `
+-- hugo.toml --
+disableKinds = ['home','rss','section','sitemap','taxonomy','term']
+
+[markup.asciidocExt]
+  backend    = 'html5s'
+  extensions = ['asciidoctor-html5s']
+
+[security.exec]
+  allow = ['^asciidoctor$']
+-- content/p1.adoc --
+---
+title: p1
+---
+
+:toc:
+
+## Section 1
+
+### Section 1.1
+
+### Section 1.2
+
+## Section 2
+-- layouts/page.html --
+{{ .TableOfContents }}
+`
+
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContentExact("public/p1/index.html", `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#_section_1">Section 1</a>
+      <ul>
+        <li><a href="#_section_1_1">Section 1.1</a></li>
+        <li><a href="#_section_1_2">Section 1.2</a></li>
+      </ul>
+    </li>
+    <li><a href="#_section_2">Section 2</a></li>
+  </ul>
+</nav>`)
+}
diff --git a/markup/asciidocext/internal/converter.go b/markup/asciidocext/internal/converter.go
index 1cc0dd541..1512609d7 100644
--- a/markup/asciidocext/internal/converter.go
+++ b/markup/asciidocext/internal/converter.go
@@ -318,7 +318,7 @@ func (a *AsciiDocConverter) extractTOC(src []byte) ([]byte, *tableofcontents.Fra
 		toVisit []*html.Node
 	)
 	f = func(n *html.Node) bool {
-		if n.Type == html.ElementNode && n.Data == "div" && attr(n, "id") == "toc" {
+		if n.Type == html.ElementNode && (n.Data == "div" || n.Data == "nav") && attr(n, "id") == "toc" {
 			toc = parseTOC(n)
 			if !a.Cfg.MarkupConfig().AsciiDocExt.PreserveTOC {
 				n.Parent.RemoveChild(n)
@@ -364,7 +364,7 @@ func parseTOC(doc *html.Node) *tableofcontents.Fragments {
 	f = func(n *html.Node, row, level int) {
 		if n.Type == html.ElementNode {
 			switch n.Data {
-			case "ul":
+			case "ul", "ol":
 				if level == 0 {
 					row++
 				}