It is located 590 feet (180 meters) off the southwest point of Caja de Muertos island and has an area of just 0.015 square miles (0.039 km2).

The island is connected to Caja de Muertos by a bank of shallow waters about 18.0 feet (5.5 meters) deep.

[8] The island is a small 31-foot flat-topped island located 200 yards off the southwestern tip of Caja de Muertos and, when viewed from a distance, Morrillito can easily be mistaken for the 170-ft steep hill (called Cerro Morrillo, Morrillo Hill) at the extreme southwestern portion of Caja de Muertos proper.

The island was designated as a nature reserve in 1980 after a meeting was held in Puerto Rico by the Puerto Rico Planning Board wherein they considered the recommendation set forth by the Coastal Management Zone Program to turn the island into a protected wilderness area.

[11] The protection is mainly due to its heavy Hawksbill sea turtle traffic which is an endangered species.