The Everglades, home to 20 endangered species, was previously on the list from 1993 to 2007 because large amounts of water were being diverted to cities.
The U.N. Educational, Scientific and Cultural Organization says in a statement the park was added again for the same reason.
UNESCO says in the Atsinanana rain forests of Madagascar, illegal logging and poaching are a threat to rare lemurs.
Being on the danger list allows the agency to allocate immediate assistance from the World Heritage Fund.
UNESCO added the two sites during a meeting Friday in Brasilia, Brazil.