| Named By: | Harry Govier Seeley in 1873 |
| Time Period: | Cretaceous, 99.6 Ma |
| Location: | England - West Melbury Marly Chalk Formation |
| Size: | Unknown due to lack of remains |
| Diet: | Piscivore |
| Fossil(s): | Fragmentary post cranial remains |
| Classification: | | Chordata | Reptilia | Ichthyosauria | Thunnosauria | |
Cetarthrosaurus is an extinct genus of ichthyosaur known from the eastern United Kingdom. It was collected from the Cambridge Greensand Formation, dating to late Albian or early Cenomanian stage, of the Early Cretaceous-Late Cretaceous boundary. Cetarthrosaurus was first named by Harry G. Seeley in 1873 and the type species is Cetarthrosaurus walkeri.