diff --git a/pyproject.toml b/pyproject.toml
index a58952a..65e9eee 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -40,7 +40,10 @@ dev = [
     "pytest-cov>=5",
     "respx>=0.21",
     "mypy>=1.10",
-    "ruff>=0.5",
+    # Kept in lockstep with the pinned ruff-pre-commit hook (.pre-commit-config.yaml).
+    # Pinned below 0.16: ruff 0.16 formats Python code blocks inside Markdown by
+    # default, which would make `ruff format --check .` reformat README.md.
+    "ruff==0.15.17",
     "pre-commit>=3.7",
     "build>=1.2",
     "twine>=5",