diff --git a/.pre-commit-hooks.yaml b/.pre-commit-hooks.yaml
index 9bea19a5..29d3bdef 100644
--- a/.pre-commit-hooks.yaml
+++ b/.pre-commit-hooks.yaml
@@ -5,6 +5,13 @@
   always_run: true
   pass_filenames: false
 
+- id: colab-toc-visible
+  name: Show the TOC sidebar in Google Colab by default
+  entry: colab-toc-visible
+  language: python
+  types:
+    - jupyter
+
 - id: fix-nbformat-version
   name: Set nbformat minor version to 4 and remove cell IDs
   entry: fix-nbformat-version
diff --git a/setup.cfg b/setup.cfg
index 3ef9f697..39be1cf1 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -74,6 +74,7 @@ dev =
 [options.entry_points]
 console_scripts =
     check-dev-files = repoma.check_dev_files:main
+    colab-toc-visible = repoma.colab_toc_visible:main
     fix-nbformat-version = repoma.fix_nbformat_version:main
     format-setup-cfg = repoma.format_setup_cfg:main
     pin-nb-requirements = repoma.pin_nb_requirements:main
diff --git a/src/repoma/colab_toc_visible.py b/src/repoma/colab_toc_visible.py
new file mode 100644
index 00000000..3c35dd3b
--- /dev/null
+++ b/src/repoma/colab_toc_visible.py
@@ -0,0 +1,54 @@
+"""Add notebook metadata to open the TOC sidebar on Google Colab.
+
+See `ComPWA/repo-maintenance#40 <https://github.com/ComPWA/repo-maintenance/issues/40>`_
+for more information.
+"""
+
+import argparse
+import sys
+from typing import Optional, Sequence
+
+import nbformat
+from nbformat import NotebookNode
+
+from .errors import PrecommitError
+from .utilities.executor import Executor
+
+
+def main(argv: Optional[Sequence[str]] = None) -> int:
+    parser = argparse.ArgumentParser(__doc__)
+    parser.add_argument(
+        "filenames",
+        nargs="*",
+        help="Paths to the notebooks of which the metadata should be updated.",
+    )
+    args = parser.parse_args(argv)
+    executor = Executor()
+    for filename in args.filenames:
+        executor(_update_metadata, filename)
+    return executor.finalize(exception=False)
+
+
+def _update_metadata(path: str) -> None:
+    notebook = open_notebook(path)
+    metadata = notebook["metadata"]
+    updated = False
+    if metadata.get("colab") is None:
+        updated = True
+        metadata["colab"] = {}
+    if not metadata["colab"].get("toc_visible"):
+        updated = True
+        metadata["colab"]["toc_visible"] = True
+    if not updated:
+        return
+    nbformat.write(notebook, path)
+    msg = f"Colab TOC is now visible for notebook {path}"
+    raise PrecommitError(msg)
+
+
+def open_notebook(path: str) -> NotebookNode:
+    return nbformat.read(path, as_version=nbformat.NO_CONVERT)
+
+
+if __name__ == "__main__":
+    sys.exit(main())