Agkistrodon piscivorus is a species of pit viper in the subfamily Crotalinae of the family Viperidae. It is one of the world's few semiaquatic vipers (along with the Florida cottonmouth), and is native to the southeastern United States. As an adult, it is large and capable of delivering a painful and potentially fatal bite. When threatened, it may respond by coiling its body and displaying its fangs. Individuals may bite when feeling threatened or being handled in any way. It tends to be found in or near water, particularly in slow-moving and shallow lakes, streams, and marshes. It is a capable swimmer and, like several species of snakes, is known to occasionally enter bays and estuaries and swim between barrier islands and the mainland.^{: 211 p.}

The generic name is derived from the Greek words ἄγκιστρον ankistron "fish-hook, hook" and ὀδών odon "tooth", and the specific name comes from the Latin pisciscode: lat promoted to code: la "fish" and vorocode: lat promoted to code: la "(I) eat greedily, devour"; thus, the scientific name translates to "hook-toothed fish-eater". Common names include cottonmouth, Northern cottonmouth, water moccasin, swamp moccasin, black moccasin, and simply viper. Many of the common names refer to the threat display, in which this species will often stand its ground and gape at an intruder, exposing the white lining of its mouth. Many scientists dislike the use of the term water moccasin since it can lead to confusion between the venomous cottonmouth and non-venomous water snakes.