diff --git a/pyproject.toml b/pyproject.toml
index 34d54fce..533cbb80 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -81,9 +81,12 @@ 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..f7687cd8
--- /dev/null
+++ b/tests/test_packaging/test_sdist_includes_tests.py
@@ -0,0 +1,25 @@
+"""Regression for https://github.com/executablebooks/markdown-it-py/issues/261."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+
+def test_flit_sdist_includes_tests_directory() -> None:
+    """sdist must ship tests/ (and tox.ini) so downstream packagers can run them."""
+    text = (Path(__file__).resolve().parents[2] / "pyproject.toml").read_text(
+        encoding="utf-8"
+    )
+    # Locate the flit sdist table without requiring tomllib (3.10 CI).
+    start = text.index("[tool.flit.sdist]")
+    rest = text[start + len("[tool.flit.sdist]") :]
+    end = rest.find("\n[")
+    block = rest if end < 0 else rest[:end]
+    assert "include = [" in block
+    assert '"tests/"' in block
+    assert '"tox.ini"' in block
+    # Still exclude heavy non-test trees.
+    assert '"docs/"' in block
+    assert '"benchmarking/"' in block
+    # Must not exclude tests anymore.
+    assert '"tests/"' not in block.split("exclude", 1)[-1]