Igapó (Portuguese pronunciation: [igaˈpɔ], from Old Tupi: "root forest") is a word used in Brazil for blackwater-flooded forests in the Amazon biome. These forests and similar swamp forests are seasonally inundated with freshwater. They typically occur along the lower reaches of rivers and around freshwater lakes. Freshwater swamp forests are found in a range of climate zones, from boreal through temperate and subtropical to tropical. In the Amazon Basin of Brazil, a seasonally whitewater-flooded forest is known as a várzea, which is similar to igapó in many regards; the key difference between the two habitats is in the type of water that floods the forest.

Igapó is primarily characterized by seasonal inundation caused by abundant rainfall; in some areas, trees can be submerged for up to 6 months of the year. These ecosystems are relatively open and feature over 30% canopy cover with an average canopy height ranging from 20–25 m (65–80 ft); sparse measurements of trees can reach 33–36 m (110–120 ft). Tree composition includes a varied mixture of leaf types and seasonality. Development of tree communities within the freshwater swamp forests rely heavily on microtopography and flooding intensity. Swamp forests can have harsh environments, in which only a few adapted species are able to sustain life or dominate over other tree species. One of the tree species mostly found in freshwater swamp forests are legume trees, in which most swamp forests have high single legume species populations. Legume trees in these forests are thought to be abundant in nitrogen fixation, which is most common in waterlogged locations and is thought to be a favorable trait when experiencing seasonal and/or constant waterlogging.

Freshwater swamp forests can be classified into two categories of forested wetlands, which are permanently and periodically flooded forests. It is possible to find these two types because of the high microtopographic variation. Due to these topographic differences, tree communities are delimited and have a distribution over small continuous areas.

These forests may be able to sustain a large number of bird, mammal, reptile, amphibian, fish and invertebrate species, however, biodiversity varies between wetland ecosystems and the species richness of the freshwater swamp forests is not entirely known. As for plant species, these forests have a lower diversity compared to other types of forests, such as the terra firme, in South America.

Igapó forests are characterized by sandy acidic soils that have a low nutrient content. The color and acidity of the water is due to soil organic molecules called humic substances (e.g. humic acid, fulvic acid) that are dissolved in the water. The acidity from the water translates into acidic soils. They are the more nutrient poor than a comparable várzea forest. They also "carry less suspended inorganic elements and contain elevated concentrations of dissolved organic material such as humic and fulvic acids". Therefore, igapó forests support comparatively less life and the environment found within these areas tend to lack species diversity and animal biomass. Várzea forest soils have high nutrient contents because they receive a transport of high sediment loads from the whitewater rivers. In contrast, igapó forests do not receive this seasonal influx of sediments which explains the nutrient poor soils. However, they do have the highest phosphorus concentrations out of comparable várzea and terra firma forest soils. Nevertheless, the seasonal inundation of the soils leads to anoxic conditions which limits plant growth.

Igapó and other flooded forests typically display a lower diversity of plants than that seen in terra firma forests. Similar to other forests found throughout the tropics, it is common to observe only a few dominant tree species Distribution of trees and other plants is highly dependent on inundation tolerances of species causing a non-random distribution of plants where more flood-tolerant species are found at lower elevations and less flood-tolerant species are found on higher ground. Fabaceae (or Leguminosae) and Euphorbiaceae are the dominant families of plants observed in igapó forest.

Tree species adapted to seasonal inundation have adapted to maximize fruit production during periods of flooding in order to take advantage of newly available seed dispersal methods. Fish consume nearly all fruit that fall into the water, and species that are unable to digest the seeds eventually excrete and disperse them into the water. Dispersal by other vectors such as birds and monkeys is secondary to that of fishes in igapó. An important factor in seed survival is the presence of seed predators. Fishes that lack the strong jaws found in characins, such as catfish, digest the fleshy material of the fruit while the seeds pass through the gut unharmed. Because many catfishes are bottom-feeders, they are critical dispersers for seeds that sink upon entering the water.

Diversity of terrestrial wildlife and fishes in igapó is highly influenced by flooding. During periods of inundation, vast numbers of aquatic species migrate into the flooded forest in search of food. Fish populations are highest during maximum fruiting periods and river dolphins and giant otters move in to prey upon them. Compared to terra firma forests, igapó features lower nutrients and favors slow-growing plants with low fleshy fruit production, which has resulted in a lower diversity and abundance of animals.