Proplanicoxa is a genus of ornithopod which existed in what is now England during the Early Cretaceous period (late Barremian stage, around 126 mya).
The holotype and only specimen of Proplanicoxa, BMNH R 8649, is composed of thirteen dorsals, a sacrum with ilia, parts of the pubis and ischium. The fossils were found in 1916 by Reginald Walter Hooley on the Isle of Wight from the upper Wessex Formation of England. BMNH R 8649 was originally assigned to Vectisaurus valdensis Hulke 1879 (=Mantellisaurus atherfieldensis) by Galton in 1976. The specimen was assigned to its own genus and species by Kenneth Carpenter and Yusuke Ishida in 2010, and the type species is Proplanicoxa galtoni. The generic name means “before” (pro in Greek) + Planicoxa in reference to the postacetabular process of the ilium trending towards the horizontal as seen even stronger in Planicoxa, and the specific name honors Peter Galton. It may be synonymous with Mantellisaurus.