diff --git a/pyproject.toml b/pyproject.toml
index a3a9308cb..9a67f5d83 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -98,6 +98,10 @@ dev = [
 ]
 test = [
   "coverage[toml]>=7",
+  # Image-comparison baselines are matplotlib-version-sensitive; matplotlib 3.11.0 (released
+  # 2026-06-12) shifts scatter/alpha rendering past the RMS tolerance. Cap for tests only
+  # (the runtime dependency stays open) until the baselines are regenerated against 3.11.
+  "matplotlib<3.11",
   "pytest>=7",
   # Just for VS Code
   "pytest-cov>=4",