| Named By: | Cabrera in 1941 |
| Time Period: | Late Cretaceous |
| Location: | Antarctica, Argentina - Paso del Sapo Formation, Chile |
| Size: | Uncertain due to incomplete remains. Skull of holotype approximately 32-33 centimetres long |
| Diet: | Filter feeder |
| Fossil(s): | Three sets of remains of partial skull material, cervical (neck) vertebrae and partial limb elements |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | Aristonectidae | |
| Also known as: | | Morturneria | |
Aristonectes (meaning 'best swimmer') is an extinct genus of plesiosaur from the Late Cretaceous of what is now South America and Antarctica. The type species is Aristonectes parvidens, first named by Cabrera in 1941.
Aristonectes has been classified variously since the 1941 description, but a 2003 review of plesiosaurs conducted by Gasparini et al. found that Aristonectes was most closely related to elasmosaurid plesiosaurs like Elasmosaurus. A similar plesiosaur, Morturneria, may be a junior synonym of Aristonectes, the study found.
Aristonectes has been recently placed within its own family, along with Tatenectes, Kaiwhekea, and Kimmerosaurus, by O'Keefe and Street (2009), as sister family of the polycotylid cryptoclidoids, but an even more recent study (Otero et al., 2014) has shown Aristonectes to be a derived elasmosaurid, thus making Aristonectidae synonymous with Elasmosauridae.