A detailed study and mapping of the Neogene volcanosedimentary rocks cropping at the Bahía San Nicolás, Baja California Sur, was carried out, allowing reconnaissance and formal description of the San Nicolás Formation (a lithostratigraphic unit constituted from older to younger by the lower Toba San Antonio, Los Volcanes, Arroyo Amarillo, and the upper El Saucito members). These units reflect syn-to-postextensional episodes associated to the Gulf of California Province that show initial high subsidence onto dry land associated with low rates of sedimentation followed by shallow marine water deposition dominated by storm and macro tidal processes. The maximum age assigned to the units is 3.3 ± 0.5 Ma.