The Islas Marías (Las Tres Marías) are a small chain of islands located in the eastern Pacific Ocean, lying 94 km west from the coast of Mexico's Nayarit state (see panoramic image) and 322 km southeast from the tip of the Baja California peninsula. The group measures 80 km in length, being orientated northwest to southeast. There are three larger islands (Maria Madre, Maria Magdalena and Maria Cleofas), one smaller island (San Juanito) as well as several smaller islets lying offshore from the larger islands.
The largest member of the group is that of the 227 km² Isla Maria Madre. It is located 5 km southeast from the small island of San Januito and 4 km northwest from Isla Maria Magdalena. Maria Madre measures 23 km in length (northwest to southeast) and is up to 12 km across. The island consists of two low and flat plains located at either end of island, which rise to a higher, highly dissected and forested central area of ravines and ridges that rises to a height of 616 m — the highest elevation to be found within the Marias group. As is generally the case among the islands of the Islas Marías group, the west coast of the island is marked by cliffs and rocky slopes, while that of the east is mostly low and sandy with several wide bays.
At 128 km² in area, Maria Magdalena is the second largest of the Islas Marías, measuring 19 km in length (east to west) and up to 9 km across. From the rugged interior (457 m) its terrain descends to the sea in steep, barren slopes and cliffs on the west and to low sandy bays on the east. Maria Cleofas is the most southerly of the islands, being located 15 km southeast from Maria Magdalena. With an area of 44 km² the island measures 10 km by 7 km, rising to an elevation of 402 m. Smallest and northernmost of the islands is the 8 km long, 3 km wide island of San Juanito.
The islands are vegetated with grasslands, scrub, deciduous and sub-deciduous tropical forest and preserve 11 endemic and restricted-range species. Fauna occurring on the islands are also important, with the Tres Marías Raccoon (Procyon insularis) and the endangered Tres Marías Cottontail Rabbit (Sylvilagus graysoni) being endemic to the islands. With rich bird and marine the life the islands are protected as part of the Islas Marías Biosphere Reserve.