Rio Branco (Portuguese pronunciation: [ˈʁi.u ˈbɾɐ̃ku], White River) is a Brazilian municipality, capital of the state of Acre. Located in the valley of the Acre River in northern Brazil, it is the most populous municipality in the state, with 387,852 inhabitants, according to 2024 IBGE estimates, almost half the state population. Rio Branco was one of the first settlements to develop in the region, being the westernmost major settlement in the country and the 4th-oldest state capital city in Northern Brazil, after Belém, Manaus and Macapá.

In 1913, it became a county. In 1920, it became the capital of the territory of Acre, and in 1962, the state capital. It is the administrative center for the economic, political and cultural region.

The capital of the state of Acre developed from the rubber plantation founded on December 28, 1882, by migrant settlers from the Northeastern Region of Brazil. Then called Seringal Volta da Empresa, it was located on the right bank of the Acre river. The Gameleira tree marks this site. This is now classified as the Second District of the city.

Almost 150 years ago, the construction of workers' shacks had begun, in lands previously occupied by the Aquiri, Canamari, and Maneteri Amazonian indigenous tribes. Years later, the same Gameleira tree survived the battles fought at Volta da Empresa, between Acre revolutionaries and Bolivian troops, during the critical period of the Acre War. As a result, Brazil acquired Acre at the beginning of the 20th century.

By August 1904, Villa Rio Branco had become the main urban center of the entire Acre valley, and was the richest and most productive in the region. During this period of the rubber boom, the streets around the Gameleira were the center of commercial and urban life in this part of the Amazon. Bars, cafes, and casinos supplied the city's nightlife; commercial representatives came here of the chief national and foreign Aviation houses that handled thousands of Contos de Réis. There were many wealthy families in the city.

The urban elite was largely composed of liberal professionals and civil servants. The political administration of the Territory was transferred to the left bank of the Acre River, because its lands were higher and not as subject to flooding as the old city centre. The early commercial zone gradually became dominated by Syrian and Lebanese immigrants. By the mid-1930s, one area was known as "Bairro Beirut" (Beirut neighborhood).

By the 1950s, this older section had become run down and known as the Second District. As a large part of its main commercial houses had transferred to the city's First District, on the left bank of the river, following the construction there of the main public offices and residences of the most important families. Today, Rio Branco is seeking to find ways to reinvent itself, through relations with neighboring countries, developing heritage and eco-tourism, building on the local fish farming industry, and so on. In spite of its remote location, Rio Branco is one of the oldest cities in the region, predating other state capitals of the Brazilian interior [pt], like Goiânia, Belo Horizonte, Porto Velho, Brasília, Palmas, and Boa Vista.

Rio Branco is located at 9° 58' 29" south and 67° 48' 36" west, at an altitude of 143 metres (469 ft) above sea level. The city has developed on both sides of the Acre River, with areas known as the First (left bank) and Second Districts. The river is crossed by six bridges, the newest bridge being the Joaquim Macedo Catwalk.