Plutoniosaurus is an extinct genus of ophthalmosaurid ichthyosaur of uncertain validity from the Early Cretaceous (late Hauterivian) of the vicinity of Ulyanovsk, European Russia.

Plutoniosaurus was named by Vladimir Efimov in 1997 based on the holotype specimen UPM 2/740, a partial skeleton containing the skull, the shoulder girdle, forelimbs and vertebrae.

[2]: 82  While agreeing with the synonymy, Christopher McGowan and Ryosuke Motani considered P. bedengensis to be a nomen dubium within the genus in 2003, too poorly known to identify as a distinct species.

[5] The following cladogram shows a possible phylogenetic position of Plutoniosaurus in Ophthalmosauridae according to the analysis performed by Zverkov and Jacobs (2020).

[6] Acamptonectes densus Mollesaurus periallus Ophthalmosaurus natans Ophthalmosaurus icenicus Gengasaurus nicosiai Nannopterygius yasykovi Nannopterygius enthekiodon Nannopterygius saveljeviensis Nannopterygius borealis Arthropterygius volgensis Arthropterygius lundi Arthropterygius thalassonotus Arthropterygius hoybergeti Arthropterygius chrisorum Brachypterygius extremus Aegirosaurus leptospondylus Muiscasaurus catheti Leninia stellans Sveltonectes insolitus Athabascasaurus bitumineus Platypterygius americanus Acuetzpalin carranzai Platypterygius sachicarum Caypullisaurus bonapartei Grendelius mordax Grendelius alekseevi Grendelius pseudoscythicus Grendelius zhuravlevi Undorosaurus kielanae Undorosaurus nessovi Undorosaurus gorodischensis Platypterygius australis Plutoniosaurus bedengensis Simbirskiasaurus birjukovi Platypterygius hercynicus Sisteronia seeleyi Platypterygius platydactylus Maiaspondylus lindoei