Lençóis Maranhenses National Park (Parque Nacional dos Lençóis Maranhenses) is a national park in Maranhão state in northeastern Brazil, just east of the Baía de São José. Protected on June 2, 1981, the 155,000 ha (380,000-acre) park includes 70 km (43 mi) of coastline, and an interior composed of rolling sand dunes. During the rainy season, the valleys among the dunes fill with freshwater lagoons, prevented from draining by the impermeable rock beneath. The park is home to a range of species, including four listed as endangered, and has become a popular destination for ecotourists.

In July 2024 the site was declared a UNESCO World Heritage Site for its exceptional beauty and the fact that it is a unique natural aspect in the world.