The St. Marys Formation is a geologic formation in Maryland and Virginia, United States. It preserves fossils dating back to the Tortonian stage of the Late Miocene epoch of the Neogene period (11 to 8 million years ago). It is the youngest Miocene formation present in the Calvert Cliffs and is part of the Chesapeake Group.
A diverse vertebrate paleofauna is known from the St. Marys Formation: