Excalibosaurus (meaning "Excalibur's lizard") is a monotypic genus of marine prehistoric reptiles (ichthyosaurs) that lived during the Sinemurian stage (approximately 196.5 ± 2 Ma to 190.8 ± 1.0 Ma (million years ago)) of the Early Jurassic period in what is now England.

The holotype, discovered in 1984 near a beach on the Somerset coast, consists of the skull, forefin, part of the pectoral girdle and some vertebrae and ribs.

The second specimen is an almost complete skeleton collected in the same area in 1996, and was purchased by the Royal Ontario Museum.

[4] Excalibosaurus is related to two other genera of ichthyosaurs, Leptonectes from the Rhaetian (Late Triassic) to the Sinemurian (Early Jurassic) of England and Eurhinosaurus from the Toarcian (Early Jurassic) of Germany.

[5][6][3] It was once thought that Excalibosaurus was a junior synonym of Eurhinosaurus,[7] but the description of the 1996 specimen show many morphological differences such as the shape of the forefin (much shorter and broader in Excalibosaurus), the slender shape of the body, that clearly differentiate the two genera.