Xenoposeidon (meaning "strange or alien Poseidon", in allusion to Sauroposeidon) is a genus of rebbachisaurid sauropod dinosaur from the Early Cretaceous of England, living about 140 million years ago.

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.

[2] Xenoposeidon's distinct suite of vertebral characteristics is unlike those found in other groups of sauropods, which differ in various proportional and structural features.

So unique is the vertebra that when Taylor and Naish attempted to classify it using a phylogenetic analysis, they found that, although a neosauropod, it didn’t fit ‘comfortably’ into any of the established groups, Diplodocoidea, Camarasauridae, Brachiosauridae and Titanosauria.

[2] Shunosaurus Barapasaurus Patagosaurus Omeisaurus Mamenchisaurus Losillasaurus Haplocanthosaurus Nigersaurus Rebbachisaurus Limaysaurus Apatosaurus Barosaurus Diplodocus Suuwassea Amargasaurus Dicraeosaurus Jobaria Camarasaurus Brachiosaurus Xenoposeidon Euhelopus Titanosauria Fossil collector Philip James Rufford discovered the vertebra that would be later named Xenoposeidon in the early 1890s.

The vertebra was probably from the Berriasian portion of the Ashdown Beds within the Hastings, although precise information about the locality and stratigraphy have been lost if such data were ever recorded.

Mike Taylor, a sauropod vertebra specialist, stumbled upon it and became interested in the unusual specimen, entering into a description of it with Darren Naish.