The El Cien Formation is a geologic formation in eastern Baja California Sur state, Mexico.
Subunits, from younger/higher to older/lower, include the Cerro Colorado Member, San Hilario Member, and Cerro Tierra Blanca Member.
It preserves fossils dating back to the Oligocene epoch of the Paleogene period and Early Miocene epoch of the Neogene period, during the Cenozoic Era.