| Named By: | Mike P. Taylor & Darren Naish in 2007 |
| Time Period: | Early Cretaceous,140 Ma |
| Location: | England, East Sussex - Hastings Bed Formation, Ashdown Beds Formation |
| Size: | Highly uncertain given the lack of fossil remains, but estimates exist depending upon what type of sauropod Xenoposeidon was. About 15 meters long if built like a brachiosaurid, alternatively 20 meters long if built like a diplodocid |
| Diet: | Herbivore |
| Fossil(s): | Partial posterior portion of a dorsal (back) vertebra |
| Classification: | | Chordata | Reptilia | Dinosauria | Saurischia | Sauropoda | |
Xenoposeidon (meaning "strange or alien Poseidon", in allusion to Sauroposeidon) is a possibly dubious genus of sauropod dinosaur from the Early Cretaceous of England, living about 140 million years ago. It is known from a single partial vertebra with unusual features, unlike those of other sauropods. This bone was first discovered in the early 1890s but received little attention until it was found by University of Portsmouth student Mike Taylor, who formally described and named it in 2007 with Darren Naish.