The Acarai Mountains are a wet, forested highland region of low mountains located in the southern part of Guyana. This range lies along the common border between Guyana and Brazil. The Acarai Mountains is one of four mountain ranges in Guyana, the others being the Imataka, Kanuku and Pakaraima mountains. The headwaters of the Essequibo River, the longest river in Guyana, and the Courantyne River, have their sources in this range. The actual source of the Essequibo was discovered in 2013 by a Guyanese-German team. The mountain range was first mentioned in 1821 by A.H. Brué as Sierra do Acaray.