The Hollywood celebrities will join about 1,000 demonstrators in Brasilia's streets Monday as they urge the government to halt the planned Belo Monte project.
Brazil's government says Belo Monte will provide much-needed clean energy for the country.
But Indian groups say they will be displaced by the dam. Environmentalists say its benefits won't make up for the damage to the jungle.
The "Avatar" director was in the Amazon last month with former U.S. Vice President Al Gore, pushing for a halt to rainforest destruction.