Puerto Rico Highway 303 (PR-303) is a rural road stretching between the municipalities of Cabo Rojo and Lajas in Puerto Rico. It begins in PR-301, in southeastern Boquerón, Cabo Rojo, passing south of the Sierra Bermeja before leading northeast, where it passes PR-305 and ending at a junction with PR-101. The stretch of the road spanning the southern border of Sierra Bermeja is popularly known as the Puerto Rico Alien Route () due to the number of UFO reports associated with the area it crosses.
The area around Sierra Bermeja, particularly along the southern edge of the mountain range, has been associated with UFOs in Puerto Rican folklore and pop culture since the second half of the 20th century. A green direction sign identifying the route as the was installed in the early 2000s. However, it was removed when the Puerto Rico Department of Transportation and Public Works rejected officially recognizing such designation on a state level. On April 9, 2005, Marcos "TurÃÂn" Irizarry Pagán, major of Lajas, proclaimed the section of PR-303 spanning the municipality between its municipal boundary with Cabo Rojo and the route's junction with PR-305. Nevertheless, the route is now promoted as a tourist attraction by the municipality of Lajas and by local businesses.