An illustrated key based on morphological, ecological and geographical characteristics is presented for the Isoëtes taxa of the Coastal Plain, Piedmont and the eastern side of the Appalachian Mountains of Alabama, District of Columbia, Delaware, southern Maryland, North Carolina, South Carolina, Florida, Georgia, Mississippi, eastern Louisiana, southeastern Tennessee and eastern Virginia. The key includes 22 species, three additional subspecies/ varieties and nine sterile hybrids. It is primarily intended to serve as an investigative tool to assist field botanists engaged in conservation, floristic and ecological studies in the Southeast.