Puerto Rico, freely associated commonwealth of the United States, composed of one large island and several small islands. Officially the Commonwealth of Puerto Rico (Spanish, Estado Libre Asociado de Puerto Rico), Puerto Rico is bordered on the north by the Atlantic Ocean; on the east by the Virgin Passage (which separates it from the Virgin Islands); on the south by the Caribbean Sea; and on the west by the Mona Passage (which separates it from the Dominican Republic). Puerto Rico became a US commonwealth on July 25, 1952.