diff --git a/pyproject.toml b/pyproject.toml
index 5ddae880..df44491f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -57,6 +57,7 @@ rtd = [
 ]
 testing = [
     "coverage",
+    "flit_core>=3.4,<5",  # lets tests/test_packaging build the sdist
     "pytest",
     "pytest-cov",
     "pytest-regressions",
@@ -81,9 +82,12 @@ markdown-it = "markdown_it.cli.parse:main"
 name = "markdown_it"
 
 [tool.flit.sdist]
+include = [
+    "tests/",
+    "tox.ini",
+]
 exclude = [
     "docs/",
-    "tests/",
     "benchmarking/"
 ]
 
diff --git a/tests/test_packaging/test_sdist_includes_tests.py b/tests/test_packaging/test_sdist_includes_tests.py
new file mode 100644
index 00000000..b16118a3
--- /dev/null
+++ b/tests/test_packaging/test_sdist_includes_tests.py
@@ -0,0 +1,28 @@
+"""Regression for https://github.com/executablebooks/markdown-it-py/issues/261."""
+
+from __future__ import annotations
+
+from pathlib import Path
+import tarfile
+
+import pytest
+
+ROOT = Path(__file__).resolve().parents[2]
+
+
+def test_sdist_contents(tmp_path: Path) -> None:
+    """The sdist must ship tests/ and tox.ini, so downstream packagers can run them."""
+    flit_sdist = pytest.importorskip("flit_core.sdist")
+    if not (ROOT / "pyproject.toml").is_file():
+        pytest.skip("not running from a source checkout")
+
+    builder = flit_sdist.SdistBuilder.from_ini_path(ROOT / "pyproject.toml")
+    with tarfile.open(builder.build(tmp_path)) as tar:
+        # strip the leading `markdown_it-<version>/` component
+        names = {name.split("/", 1)[1] for name in tar.getnames() if "/" in name}
+
+    assert "markdown_it/__init__.py" in names
+    assert "tests/test_api/test_main.py" in names
+    assert "tox.ini" in names
+    # heavy, non-essential trees are still excluded
+    assert not [name for name in names if name.startswith(("docs/", "benchmarking/"))]