Hippocastanoideae is a subfamily of flowering plants in the soapberry family Sapindaceae. The group was formerly treated as the separate families Aceraceae and Hippocastanaceae. Molecular phylogenetic research by Harrington et al. (2005) has shown that while both the Aceraceae and Hippocastanaceae are monophyletic in themselves, their removal from the Sapindaceae sensu lato would leave Sapindaceae sensu stricto as a paraphyletic group, particularly with reference to the genus Xanthoceras.