Archaeonectrus is an extinct genus of pliosaur from the Early Jurassic (Sinemurian) of what is now southeastern England. The type species is Archaeonectrus (originally "Plesiosaurus") rostratus, first named by Sir Richard Owen in 1865, which was moved to its own genus by N.I. Novozhilov in 1964. It was a relatively small plesiosaur, measuring 3.4–3.67 m (11.2–12.0 ft) long.

The cladogram below shows Archaeonectrus phylogenetic position among other plesiosaurs, following Benson et al. (2012).