Phrynops williamsi, also known commonly as Williams´ side-necked turtle, Williams´ South American sideneck turtle, William´s South American side-necked turtle, William´s toadhead turtle, and Williams´ toadhead turtle, is a species of turtle in the family Chelidae. The species is endemic to South America.

Etymology: The specific name, williamsi, is in honor of American herpetologist Ernest E. Williams.

Geographic range: P. williamsi is found in southeastern Brazil, Uruguay, and Argentina.

Habitat: The preferred natural habitat of P. williamsi is freshwater wetlands, at altitudes of .