|Elevation||908 ft (277 m)|
|Etymology||Spanish for vermilion mountain range|
|Location||Cabo Rojo, Puerto Rico|
Sierra Bermeja is a mountain range in southwestern Puerto Rico. It forms the southern boundary of the Lajas Valley and it is bordered by La Parguera in the Caribbean Sea coast. It extends from the municipality of Cabo Rojo in the west to Lajas and Guanica in the east. It consists of a combination of volcanic rocks, completely crossed by serpentinite and amphibolite faults, which could well be the oldest known rock in Puerto Rico. The mountains were used by the Taíno people as refuge from the Spanish, and then by smugglers during the Spanish colonization.
The Sierra Bermeja is the nesting place of many species that feed in the Laguna Cartagena. These hills are among a few forested areas of the southwestern coastal plain, characterized by its aridity. As a result, the Sierra provides the requirements for a large quantity of wildlife established in the area, many of it endangered.
In 2015, volunteers searched the area for a meteorite that, according to NASA, may have landed in Sierra Bermeja. Development in the region had become a threat to the area, despite being one of the most important natural areas in Puerto Rico.  In June 2018, funds were earmarked for universities in Puerto Rico to use for research of the Sierra Bermeja region. Local folklore surrounding the area says there is a secret UFO base somewhere in the Sierra Bermeja.
- JE, Aukema; Anadón-Irizarry, V.; Tossas, AG; Carlo, TA. "A Call to Protect Sierra Bermeja for Future Generations". National Center for Ecological Analysis. Sociedad Ornitológica Puertorriqueña, San Juan, Puerto Rico. ISBN 0-9778323-1-7. Retrieved 1 November 2019.
- "No aparece el meteorito que cayó en Lajas". Primera Hora (in Spanish). July 2, 2015.
- "Nearly $1.7 million assigned for access to STEM education, fostering agriculture and natural resource protection". Caribbean Business. June 13, 2018.
- "¿Extraterrestres visitan Puerto Rico?". Primera Hora. June 13, 2016.
- "Leyendas urbanas poco conocidas en Puerto Rico". Metro (in Spanish).
- "Rare plants protected, propagated via Partners program in Puerto Rico". Southeast Region of the U.S. Fish and Wildlife Service. Retrieved 2021-08-29.