The geography of Puerto Rico consists of an archipelago located between the North Atlantic Ocean and the Caribbean Sea, east of Hispaniola, west of the Virgin Islands, north of Venezuela, and south of the Puerto Rico Trench, the deepest point in the entire Atlantic. As the easternmost and smallest of the Greater Antilles, the main island of Puerto Rico is about 178 kilometers long and 65 kilometers wide. With a land and internal coastal water area of 9,100 square kilometres (3,500 sq mi), it is the 4th largest island in the Caribbean and 81st largest island in the world. Various smaller islands and cays, including Vieques, Culebra, Palomino, Mona, Desecheo, and Caja de Muertos, comprise the rest of the archipelago, with only Culebra and Vieques being inhabited.

Mona 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, a strait between the Dominican Republic and Puerto Rico, 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.