| Named By: | Schwermann & Sander in 2011 |
| Time Period: | Early Jurassic, Pliensbachian |
| Location: | Germany, Westfalen |
| Size: | Estimated at 4.5 meters long |
| Diet: | Probably a piscivore |
| Fossil(s): | Almost complete and still articulated skeleton, although the skull and upper cervical (neck) vertebrae are missing |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | Plesiosauridae | |
Westphaliasaurus is an extinct genus of plesiosaurid from Lower Jurassic (Pliensbachian stage) deposits of Westphalia, northwestern Germany. It is known from a nearly complete and articulated skeleton missing the skull and about 38% of the upper neck vertebrae. It was found by Sonke Simonsen, an amateur paleontologist, in 2007 from the Hoxter district near Bielefeld, in the east of North Rhine-Westphalia, Germany. It was first named by Leonie Schwermann and Martin Sander in 2011 and the type species is Westphaliasaurus simonsensii. The generic name is derived from the latinized name for Westfalen, Westphalia and lizard, saurus. The specific name honors Sonke Simonsen. Estimates suggest that it was 4.5 metres (15 ft) in length.