The Puerto Rico Trench is an oceanic trench located on the bound­ary between the Caribbean Sea and the Atlantic Ocean. The trench is asso­ci­ated with a com­plex tran­si­tion between the sub­duc­tion zone to the south along the Lesser Antilles island arc and the major trans­form fault zone or plate bound­ary that extends west between Cuba and His­pan­iola through the Cay­man Trench to the coast of Cen­tral Amer­ica. Sci­en­tific stud­ies have con­cluded that an earth­quake occur­ring along this fault zone could gen­er­ate a sig­nif­i­cant tsunami.

The island of Puerto Rico lies imme­di­ately to the south of the fault zone and the trench. The trench is 800 kilo­me­tres (500 mi) long and has a max­i­mum depth of 8,605 metres (28,232 ft) at Mil­wau­kee Deep, which is the deep­est point in the Atlantic Ocean.
The Puerto Rico Trench is located at a bound­ary between two plates that pass each other along a trans­form bound­ary with only a small com­po­nent of sub­duc­tion. The Caribbean Plate is mov­ing to the east while the North Amer­i­can Plate is mov­ing to the west. The North Amer­i­can Plate is being sub­ducted by the Caribbean Plate to the south­east of the trench. This sub­duc­tion zone explains the pres­ence of active vol­ca­noes over the south­east­ern part of the Caribbean Sea. Vol­canic activ­ity is fre­quent along the island arc south­east from Puerto Rico to the coast of South America.

Puerto Rico, the United States Vir­gin Islands, British Vir­gin Islands and the Domini­can Repub­lic do not have active vol­ca­noes; how­ever they are at risk from earth­quakes and tsunamis. The Puerto Rico Trench is capa­ble of pro­duc­ing earth­quakes higher than Mag­ni­tude 8.0.