diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9d269fc123..ebff906cd1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -35,6 +35,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - CSS error reporting will no longer provide links to the files in question https://github.com/Textualize/textual/pull/3582
 - inline CSS error reporting will report widget/class variable where the CSS was read from https://github.com/Textualize/textual/pull/3582
 - Breaking change: Setting `Select.value` to `None` no longer clears the selection (See `Select.BLANK` and `Select.clear`) https://github.com/Textualize/textual/pull/3614
+- Markup in markdown headings is now escaped when building the TOC https://github.com/Textualize/textual/issues/3689
 
 
 ## [0.41.0] - 2023-10-31
diff --git a/src/textual/widgets/_markdown.py b/src/textual/widgets/_markdown.py
index ac6de4f4bc..96b81cc6eb 100644
--- a/src/textual/widgets/_markdown.py
+++ b/src/textual/widgets/_markdown.py
@@ -937,7 +937,8 @@ def set_table_of_contents(self, table_of_contents: TableOfContentsType) -> None:
                     node.allow_expand = True
                 else:
                     node = node.add(NUMERALS[level], expand=True)
-            node.add_leaf(f"[dim]{NUMERALS[level]}[/] {name}", {"block_id": block_id})
+            node_label = Text.from_markup(f"[dim]{NUMERALS[level]}[/] ") + Text(name)
+            node.add_leaf(node_label, {"block_id": block_id})
 
     async def _on_tree_node_selected(self, message: Tree.NodeSelected) -> None:
         node_data = message.node.data
diff --git a/tests/test_markdownviewer.py b/tests/test_markdownviewer.py
index 27ccf0da99..8d94d4b946 100644
--- a/tests/test_markdownviewer.py
+++ b/tests/test_markdownviewer.py
@@ -1,10 +1,12 @@
 from pathlib import Path
 
 import pytest
+from rich.text import Text
 
+import textual.widgets._markdown as MD
 from textual.app import App, ComposeResult
 from textual.geometry import Offset
-from textual.widgets import Markdown, MarkdownViewer
+from textual.widgets import Markdown, MarkdownViewer, Tree
 
 TEST_MARKDOWN = """\
 * [First]({{file}}#first)
@@ -45,8 +47,12 @@ async def test_markdown_file_viewer_anchor_link(tmp_path, link: int) -> None:
 
 
 class MarkdownStringViewerApp(App[None]):
+    def __init__(self, markdown_string: str) -> None:
+        self.markdown_string = markdown_string
+        super().__init__()
+
     def compose(self) -> ComposeResult:
-        yield MarkdownViewer(TEST_MARKDOWN.replace("{{file}}", ""))
+        yield MarkdownViewer(self.markdown_string)
 
     async def on_mount(self) -> None:
         self.query_one(MarkdownViewer).show_table_of_contents = False
@@ -57,8 +63,27 @@ async def test_markdown_string_viewer_anchor_link(link: int) -> None:
     """Test https://github.com/Textualize/textual/issues/3094
 
     Also https://github.com/Textualize/textual/pull/3244#issuecomment-1710278718."""
-    async with MarkdownStringViewerApp().run_test() as pilot:
+    async with MarkdownStringViewerApp(
+        TEST_MARKDOWN.replace("{{file}}", "")
+    ).run_test() as pilot:
         # There's not really anything to test *for* here, but the lack of an
         # exception is the win (before the fix this is testing it would have
         # been FileNotFoundError).
         await pilot.click(Markdown, Offset(2, link))
+
+
+@pytest.mark.parametrize("text", ["Hey [[/test]]", "[i]Hey there[/i]"])
+async def test_headings_that_look_like_they_contain_markup(text: str) -> None:
+    """Regression test for https://github.com/Textualize/textual/issues/3689.
+
+    Things that look like markup are escaped in markdown headings in the table of contents.
+    """
+
+    document = f"# {text}"
+    async with MarkdownStringViewerApp(document).run_test() as pilot:
+        assert pilot.app.query_one(MD.MarkdownH1)._text == Text(text)
+        toc_tree = pilot.app.query_one(MD.MarkdownTableOfContents).query_one(Tree)
+        # The toc label looks like "I {text}" but the I is styled so we drop it.
+        toc_label = toc_tree.root.children[0].label
+        _, text_label = toc_label.divide([2])
+        assert text_label == Text(text)