The Atlantic Forest (Portuguese: Mata Atlântica), also called Missionary rainforest (Spanish: Selva Misionera), is a moist broadleaf forest that extends along the Atlantic coast of Brazil from Rio Grande do Norte state in the northeast to Rio Grande do Sul state in the south and inland as far as Paraguay and the Misiones Province of Argentina.

The Atlantic Forest has ecoregions within the following biome categories: seasonal moist and dry broad-leaf tropical forests, tropical and subtropical grasslands, savannas, and shrublands, and mangrove forests. The Atlantic Forest is characterized by a high biodiversity and endemism.

It was the first environment that the Portuguese colonists encountered over 500 years ago, when it was thought to have had an area of 1,000,000–1,500,000 km² (390,000–580,000 sq mi), and stretching an unknown distance inland, making it, back then, the second largest rainforest on the planet, only behind the Amazon rainforest. Over 85% of the original area has been deforested, threatening many plant and animal species with extinction.

The Atlantic Forest region includes forests of several variations:

The Atlantic Forest is unusual in that it extends as a true tropical rain forest to latitudes as far as 28°S. This is because the trade winds produce precipitation throughout the southern winter. In fact, the northern Zona da Mata of northeastern Brazil receives much more rainfall between May and August than during the southern summer. The geographic range of Atlantic Forest vary depending on author or institution that published them. Information on four most important boundaries as well as their union and intersection was reviewed in 2018.

The Atlantic Forest mainly covers regions of eastern Brazil (92% of the total area), but also reaches eastern Paraguay (6%) and northeastern Argentina (2%).

During glacial periods in the Pleistocene, the Atlantic Forest is known to have shrunk to extremely small fragmented refugia in highly sheltered gullies, being separated by areas of dry forest or semi-deserts known as caatingas. Some maps even suggest the forest actually survived in moist pockets well away from the coastline where its endemic rainforest species mixed with much cooler-climate species.^{[citation needed]} Unlike refugia for equatorial rainforests, the refuges for the Atlantic Forest have never been the product of detailed identification.^{[citation needed]}

Despite having only 28% of native vegetation cover remaining, the Atlantic Forest remains extraordinarily lush in biodiversity and endemic species, many of which are threatened with extinction. Approximately 40 percent of its vascular plants and up to 60 percent of its vertebrates are endemic species, meaning they are found nowhere else in the world. The official threatened species list of Brazil contains over 140 terrestrial mammal species found in Atlantic Forest. In Paraguay the Atlantic Forest has been heavily impacted in recent years. In Paraguay there are 35 species listed as threatened, and 22 species are listed as threatened in the interior portion of the Atlantic Forest of Argentina. Nearly 250 species of amphibians, birds, and mammals have become extinct due to the result of human activity in the past 400 years. Over 11,000 species of plants and animals are considered threatened today in the Atlantic Forest. Over 52% of the tree species and 92% of the amphibians are endemic to this area. The forest harbors around 20,000 species of plants, with almost 450 tree species being found in just one hectare in some locations.