The Santa Cruz Mudstone is a geologic formation in California.

The siliceous organic mudstones of the formation were deposited in deep water and fluvial environments.

The Santa Cruz Mudstone was formerly considered part of the Monterey Formation.

The formation preserves bivalve and echinoid fossils as well as vertebrates of Parabalaenoptera baulinensis and Otodus megalodon.

The formation dates back to the Late Miocene (Tortonian to Messinian) period.