Umoonasaurus is an extinct genus of plesiosaur belonging to the family Leptocleididae. This genus lived approximately 115 million years ago (Aptian-Albian) in shallow seas covering parts of what is now Australia. It was a relatively small animal around 2.5 m (8 ft) long. An identifying trait of Umoonasaurus is three crest-ridges on its skull.
Umoonasaurus is known from a relatively complete skeleton preserved as opal from Coober Pedy in South Australia. This specimen was prepared by Paul Willis at the Australian Museum, Sydney, and became the focus of a nationwide fundraising appeal coordinated by ABC TV's Quantum program.