The Brazil-nut poison frog (Adelphobates castaneoticus) is a species of frog in the family Dendrobatidae. It is endemic to the state of Pará in Brazil. Its natural habitats are tropical moist lowland forests and intermittent freshwater marshes. It is threatened by habitat loss. The frog is believed to have received its common name from the fact that its tadpoles sometimes develop in the hard capsules of the Brazil nut tree, which are common in its range. The nuts fall to the forest floor where they are broken open by agoutis and other animals seeking the seeds, and empty husks fill with water.