diff --git a/pyproject.toml b/pyproject.toml
index 4c4735e5..58115f86 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -110,6 +110,9 @@ ignore-words-list = "gage,gages"
 
 [tool.pytest.ini_options]
 addopts = "-v --import-mode=importlib --cov=swmmanywhere --cov-report=html --doctest-modules --ignore=swmmanywhere/logging.py"
+markers = [
+  "downloads: mark a test as requiring downloads",
+]
 
 [tool.mypy]
 disallow_any_explicit = false