westernmost estado (state) of Brazil. Acre covers the southwesternmost part of Brazil's Hiléia (Hylea), the forest zone of the Amazon River basin. Bounded north by Amazonas state, it has western and southern frontiers with Peru and southeastern with Bolivia. The capital is Rio Branco on the Rio Acre in the eastern part of the state. The state's name is derived from that of the Rio Acre, which seems to be of Indian origin. Covered by tropical rainforest, Acre produces the highest-quality rubber in Brazil.

Portuguese explorers, making their way through the immense Amazon forest from the Atlantic estuary of the Amazon River, did not reach Acre before the middle decades of the 18th century, when there were no settled inhabitants but only roving bands of Indians. Under the Brazilian Empire, more expeditions began to penetrate the territory in the 1850s and '60s; and although the whole area was ceded by Brazil to Bolivia in 1867 (by the Treaty of Ayacucho), the rubber boom of the following decades attracted more and more immigrants from northeastern Brazil. In 1899, during a local revolution, an independent Republic of Acre was proclaimed by Luís Gálvez Rodríguez, a Spanish adventurer, but this regime was short-lived. After further vicissitudes, negotiations sponsored by Brazil's foreign minister culminated in 1903 in the Treaty of Petropólis, whereby Acre was reincorporated with Brazil. The frontier with Peru was agreed upon in 1909. Organized at first as a territory, Acre achieved statehood in 1962.

With an average elevation of 600 feet (183 metres) above sea level, the land slopes gently down toward the Amazonian Plain from the higher ground in the west and south. An average annual temperature of 77 °F (25 °C) and an annual rainfall of 79–98 inches (2,000–2,500 mm) makes the climate warm and humid. The forest is traversed by headstreams of two of the Amazon's major tributaries, the Juruá (Juruá River) and the Purus (Purus River). The principal trees of the forest are rubber and Brazil nut. The fauna include peccaries (piglike animals), red deer, capybaras (rodents with no tail and partly webbed feet), agoutis (short-haired, short-eared rodents), and tapirs (large, hoofed quadrupeds).

The local Indian tribes are few and small, yet a large proportion of the state is set aside for Indian reservations and parks. Most of Acre's population consists of immigrants, or descendants of immigrants, from northeastern Brazil, the decided majority of them Afro-Brazilian. Many people still live on the rivers and tracks that give access to the raw materials of the forests, but the majority dwell in urban areas. The largest city is Rio Branco, which contains about two-fifths of the state's population. Other major towns are Cruzeiro do Sul, Sena Madureira, Tarauacá, and Feijó. The common language is Portuguese, the predominant religion Roman Catholicism. Amoebic dysentery, malaria, and leprosy are the major endemic diseases.

Agriculture, which is often of the subsistence type, is concerned with short-cycle crops, chiefly cassava (manioc), corn (maize), and bananas. Zebus (humped oxen) are raised on the open-range system, and pigs and chickens are bred quite widely.

Rivers are the main channels of communication, but a serviceable highway runs across most of the state. Rio Branco is connected with Manaus, capital of Amazonas, by the Rio Acre-Purus-Amazon linkage and with Brasília—the federal capital—by a land highway; there are also air services connecting Rio Branco with both places. Area 58,912 square miles (152,581 square km). Pop. (2006 est.) 686,652.