Barcelos (also Barcellos), formerly Mariuá, is a municipality located in the state of Amazonas, northern Brazil.

Since 1994, Barcelos has been the host of an annual festival celebrating ornamental fish,[4] which is a significant source of income for the region.

Project Piaba uses Barcelos as the starting point for their annual research expedition into the Rio Negro area.

The municipality area is 122,476 square kilometres (47,288 sq mi), making it the second largest municipality in Brazil (behind Altamira, Pará), equivalent in size to New York state in the United States and slightly larger than North Korea.

[10] The climate reflects its vegetation being tropical rainforest or equatorial (Köppen: Af), like the rest of the Amazon Basin.