The Hargreaves and Samani model was used to estimate monthly potential evapotranspiration (PET) for Central Aguirre, Fortuna and Lajas substations and Magueyes Island located on the south coast of Puerto Rico. The model uses maximum, minimum and average temperatures. Daily PET varies from 3.68 to 5.37 mm/day in the region with minimum in December and maximum in July. Annual PET was 1613.3 mm/year for Central Aguirre, 1653.5 for Fortuna, 1846.9 for Lajas, 1857.9 for Magueyes Island, with a regional average of 1704.6. These PET values can be used to estimate crop water requirements for vegetables and fruits, to plan irrigation and water management projects, and to schedule irrigation in the semiarid region of Puerto Rico.