Isla Cerralvo is located within the Sea of Cortez near the southern end of the gulf coast of Baja California and provides an outstanding opportunity to study key geologic relationships relevant to the formation of Mesozoic batholiths that make the crystalline backbone of the Baja California peninsula, as well as the Late Miocene-Pliocene rift history associated with the Pacific-North American plate boundary jump into the Gulf of California. In spite of its strategic location, Isla Cerralvo has received scant geologic study and its geology and relationship to the rest of the peninsula has been largely overlooked. The island occupies the footwall of San Jose del Cabo fault. This major down-to-the-east normal fault exhumed the Los Cabos Block, along with Isla Cerralvo, beginning in the Middle Miocene synchronous with proto-Gulf of California extension. Isla Cerralvo is composed primarily of Mesozoic plutonic and metamorphic rocks cut by mafic dikes. Small areas of sedimentary and volcanic rocks of presumed Tertiary age occur in the west-central and northern regions of the island and carbonate cemented terraces of Pliocene age occur at the southern end of the island. Mesozoic basement is part of the larger Los Cabos Block, which is a major crystalline massif that dominates the southern tip of the Baja California peninsula. Two main Cretaceous granodiorite units are recognized on the basis of age, lithology, whole rock geochemistry, structural/metamorphic differences, and the presence or absence of mafic dikes. Concordant 40Ar-39Ar hornblende and biotite ages and zircon U-Pb ages document ~100-91 Ma crystallization ages for Cretaceous plutonic units. The two main Cretaceous intrusive units on Isla Cerralvo are termed Kgd1 and Kgd2; with field relations and isotopic ages suggesting the Kgd1 suite is slightly older (~2-3 Ma) than Kgd2. The granodiorite map units intrude a package of greenschist to amphibolite facies quartzofeldspathic schist. Based upon observed mineral assemblages, the overall depth of metamorphism appears comparatively shallow (~4-8 km). The south end of the island is comprised of intermingled S-type granite, orthogneiss and amphibolite grade flysch-like metasedimentary rock collectively termed here the ‘Montana Complex’. Zircon U-Pb ages of 160.8 ± 1.2 Ma and 165.2 ± 0.5 Ma for S-type granite and a biotite orthogneiss, respectively, document for the first time the presence of Jurassic plutonic rocks in the Los Cabos block. At the north end of the island, a welded rhyolitic ignimbrite was deposited across a thick layer of weakly indurated conglomerate. The clasts in this conglomerate consist almost entirely of schist that appears to be derived from in situ underlying prebatholithic metasedimentary basement. Steeply dipping normal faults truncate the schist/conglomerate/rhyolite sequence on opposing sides and define a small graben structure. These volcanic deposits are presumed to correlate to the Miocene Comondu Group, which surrounds the La Paz area across the channel from Isla Cerralvo. Fossiliferous sandstone/limestone/marl sequences along the west-central coast occupy the hanging wall of a west-dipping normal fault antithetic to the San Jose del Cabo Fault. Pliocene limestone/sandstone terrace deposits have been uplifted along the south coast of the island.