Phrynops williamsi, also known commonly as Williams' side-necked turtle, Williams' South American sideneck turtle, William's [sic] South American side-necked turtle, William's [sic] toadhead turtle, and Williams' toadhead turtle, is a species of turtle in the family Chelidae. The species is endemic to South America.
The specific name, williamsi, is in honor of American herpetologist Ernest E. Williams.
P. williamsi is found in southeastern Brazil, Uruguay, and Argentina.
The preferred natural habitat of P. williamsi is freshwater wetlands, at altitudes of 80–1,000 m (260–3,280 ft).