estado (state), west-central Brazil, bordering Bolivia to the south and west, Amazonas state to the north, and Mato Grosso state on the east. Formerly a part of Amazonas, it was established in 1943 as Guaporé territory, which was renamed in 1956 after Marshal Cândido Mariano da Silva Rondon, an explorer and protector of Indians. Rondônia became a state in 1982.

Much of Rondônia's area is rainforest. The region was important during the Amazon rubber boom but declined after the collapse of the industry in 1912. The Madeira-Mamoré Railway, which once operated between Pôrto Velho (the state capital, in the extreme northwestern part of the state) and Guajará-Mirim, was built to transport rubber. Rubber is still exported, along with timber, meat, and tin. cassiterite (the major ore of tin) mining expanded rapidly in the 1980s and '90s, drawing large-scale population influx. The cassiterite mine at Ariquemes is one of the largest in the world. Gold is also found nearby. Cattle, corn, and rice are grown, and lumber is a significant export. The Rondônia hydroelectric project, built at the Samuel Falls, supplies energy for the mahogany and tin industries. Transportation on the Madeira (Madeira River), Mamoré (Mamoré River), and Guaporé (Guaporé River) rivers and their affluents is supplemented by highways and air service from Pôrto Velho, Guajará-Mirim, and Ariquemes. Area 91,729 square miles (237,576 square km). Pop. (2006 est.) 1,562,417.