The US is the closest country.
Dominican Republic
The Dominican Republic is the closest country to Puerto Rico.
Puerto Rico is an island in the Caribbean and a territory of the United States, so the closest country to Puerto Rico is the Dominican Republic. It lies to the west of Puerto Rico just across the Mona Strait. The next closest country to Puerto Rico is Great Britain! The British Virgin Islands are to the East of the US Virgin Island which in turn are about 60 miles off Puerto Rico's east coast.
Mercedita Airport at Ponce Puerto Rico.
Puerto Rico is an island and is surrounded by the Atlantic Ocean and Caribbean Sea. The closest country to Puerto Rico is the Dominican Republic about 60 miles west. The St. Thomas, US Virgin Islands is about 30 miles to the East of Puerto Rico.
The description of "opposite" is somewhat ambiguous. The nearest foreign territory to Puerto Rico is Charlotte Amalie, US Virgin Islands, about 47 miles due east of PR's largest island. Due west about 75 miles lies the eastern tip of Cuba.
Puerto Rico
Puerto Rico.
It's not US territory is a part but it is a country dumb
St. Thomas is the closest of the Virgin Islands to Puerto Rico. It is only about 15 miles east of the Puerto Rican outlying island of Culebra and about 35 miles east of the Island of Puerto Rico.
Florida