| Named By: | Harry Govier Seeley in 1874 |
| Time Period: | Early Jurassic, Toarcian |
| Location: | England, Northamptonshire and Yorkshire - Whitby Mudstone Formation |
| Size: | Larger individuals about 7 meters long |
| Diet: | Carnivore |
| Fossil(s): | Many specimens, some almost complete making Rhomaleosaurus one of the best preserved marine reptiles |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | Pliosauroidea | Rhomaleosauridae | |
| Also known as: | | Plesiosaurus cramptoni | Plesiosaurus propinquus | Rhomaleosaurus thorntoni | Thaumatosaurus cramptoni | Thaumatosaurus propinquus | |
Rhomaleosaurus (meaning "strong lizard") is an extinct genus of Early Jurassic (Toarcian age, about 183 to 175.6 million years ago) rhomaleosaurid pliosauroid known from Northamptonshire and from Yorkshire of the United Kingdom. It was first named by Harry Seeley in 1874 and the type species is Rhomaleosaurus cramptoni. It was one of the earliest large marine reptile predators which hunted in the seas of Mesozoic era. Its length was about 7 m (23 ft) long. Like other pliosaurs, Rhomaleosaurus fed on ichthyosaurs, ammonites and other plesiosaurs.