The Islas Marías (“Mary Islands”) are an archipelago of four islands that belong to Mexico. They are located in the Pacific Ocean, some 100 km (62 mi) off the coast of the state of Nayarit. They are part of the municipality (municipio) of San Blas, Nayarit. As of 2011, the islands are still being used as a penal colony, containing the Islas Marias Federal Prison.
The islands have an aggregate area of 244.970 km². 1,116 people lived on Isla María Madre as of the census of 2005. The other islands are uninhabited. The main settlement is Puerto Balleto, with a population of 602.
Isla María Madre is the largest of the islands, with a surface area of 145.282 km². It houses the Islas Marías Federal Prison which was established there in 1905. The next two largest islands are Isla María Magdalena (70.440 km²) and, further south, Isla María Cleofas (19.818 km²).
A smaller island, Isla San Juanito, with an area of 9.105 km² lies off the north coast of Isla María Madre. In 2010 the archipelago was designated the Islas Marías Biosphere Reserve by UNESCO.