The following is a list of ecoregions in the United States as identified by the World Wide Fund for Nature (WWF). The United States is a megadiverse country with a high level of endemism across a wide variety of ecosystems.

Terrestrial ecoregions

The United States is unique among countries in that its terrestrial ecoregions span three biogeographic realms: the Nearctic, Neotropical, and Oceanian realms.

Fifty states

Alaska is the most biodiverse state with 15 ecoregions across three biomes in the same realm. California comes in a close second with 13 ecoregions across four biomes in the same realm. By contrast, Rhode Island is the least biodiverse with just one ecoregion—the Northeastern coastal forests—encompassing the entire state. The terrestrial ecoregions of the 50 states of the United States are as follows:

Five inhabited territories

The ecoregions of the 5 inhabited territories of the United States are as follows:

Marine ecoregions

The marine ecoregions of the 50 states of the United States are as follows: