Visit Praia do Pinho in the state of Santa Catarina. It's been frequently listed as one of the best nude beaches in the world since the late 1980's. In 2007, it will host two large Brazilian Naturalist events which are sure to increase its popularity. To find this beach, head to Cambriu on the Interpraias Highway.
Check out the other beaches in Brazil that allow nude sunbathing. These include Galheta in Florianopolis, Sao Luis, Porto Allegre, Abrico and Colina do Sol.
Decide what you're looking for in a beach. If you want absolute peace and quiet, head to one that doesn't have many amenities and visit during the off-season. However, if you want action, vendors and extras such as toilets and showers, try Rio de Janeiro.