The Plain-bellied Water Snake (Nerodia erythrogaster) is a common species of mostly aquatic, non-venomous, colubrid snake found in the United States.

This species ranges through much of the southeastern United States, from Michigan to Delaware in the north, and Texas to northern Florida in the south, but it is absent from the Florida peninsula and the higher elevations of the Appalachian Mountains. They are almost always found near a permanent water source, a lake, stream, pond or other slow moving body.

Adults are 24-40 inches (76-122 cm) long. It gets its common name, because it has no patterning on its underside. Species can vary in color from brown, to grey, to olive green, with dark colored blotching down their back, and an underside that is yellow, brown, red or green. It is quick to vigorously defend itself by biting repeatedly and its mouth has a white interior, resulting in it being misidentified frequently as the venomous cottonmouth.

This species bears live young (ovoviviparous), like other North American water snakes and Garter snakes. In North Carolina and Georgia, the Plain-bellied water snake breeds from April to June, and batches of 5-27 young are born in August to October. Subspecies

There are six recognized subspecies of N. erythrogaster:

   * Plainbelly Water Snake, Nerodia erythrogaster alta (Conant, 1963)
   * Bogert's Water Snake, Nerodia erythrogaster bogerti (Conant, 1953)
   * Redbelly Water Snake, Nerodia erythrogaster erythrogaster (Forster, 1771)
   * Yellowbelly Water Snake, Nerodia erythrogaster flavigaster (Conant, 1949)
   * Copperbelly Water Snake, Nerodia erythrogaster neglecta (Conant, 1949)