Açu is a municipality in the state of Rio Grande do Norte in Brazil. The population is 58,384 in an area of 1303 km². The municipality contain…Açu is a municipality in the state of Rio Grande do Norte in Brazil. The population is 58,384 in an area of 1303 km². The municipality contains the Açu National Forest, a 218 hectares sustainable use conservation unit that was originally established as a forest park in 1950. The Barragem Armando Ribeiro Gonçalves, a reservoir on the Piranhas River, is partly located in the municipality.