Piranha is a 1978 American horror film directed and co-edited by Joe Dante from a screenplay by John Sayles, based on a story by Richard Robinson and Sayles. The film stars Bradford Dillman, Heather Menzies, Kevin McCarthy, Keenan Wynn, Barbara Steele, and Dick Miller. It tells the story of a river being infested by lethal, genetically altered piranha, threatening the lives of the local inhabitants and the visitors to a nearby summer resort.

Executive produced by Roger Corman, Piranha is the first installment in a series of low-budget B movies inspired by the film Jaws (1975), which had been a major success for Universal Pictures and director Steven Spielberg. Initially, Universal had considered obtaining an injunction to prevent Piranha being released, particularly as they had released Jaws 2 the same summer, but the lawsuit was dropped after Spielberg himself gave the film a positive comment in advance. Released on August 3, 1978, the film was a commercial success and later became a cult film. Piranha was followed by a sequel, Piranha II: The Spawning (1982), and two remakes, one in 1995 and another in 2010, which spawned its own sequel in 2012.