Mona Island is the third-largest island of the Puerto Rican archipelago, after the main island of Puerto Rico and Vieques. It is the largest of three islands in the Mona Passage, the strait between Hispaniola and Puerto Rico, with the others being Monito Island and Desecheo Island. It measures about 7 miles by 4 miles, and lies 41 mi (66 km) west of Puerto Rico, of which it is administratively a part. It is one of two islands that make up the Isla de Mona e Islote Monito barrio in the municipality of Mayagüez.