Lençóis Maranhenses National Park 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 due to 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.

National Parks are a legally-defined type of protected area of Brazil. The first parks were created in the 1930s with other parks being gradually added, typically protecting a natural monument such as a waterfall or gorge near to a coastal population centre. At least two early parks were later submerged by hydroelectric reservoirs. The first park in the Amazon rainforest was inaugurated in 1974. Today the national parks cover a huge area, particularly in the Amazon. However, many of them suffer from outstanding claims for compensation from former owners or users of the land, and many lack the management plans, physical infrastructure and personnel needed to support public visits. The responsible government agency does not have the capacity to provide services such as food and drink, souvenir sales and guided tours, and bureaucracy has delayed letting the private sector bid on providing such services.