We describe an exceptionally well preserved skull of a small anthracothere from the late middle Eocene of the Pondaung Formation that can be confidently attributed to Siamotherium pondaungensis, a species that was first diagnosed on the basis of a fragmentary maxilla preserving two upper molars. The new material confirms that Siamotherium pondaungensis is an anthracothere and not a helohyid as repeatedly suggested. According to its dentition and skull structure, Siamotherium pondaungensis was most likely a terrestrial, open-forest animal with an omnivorous diet that showed no significant adaptation to folivory. A phylogenetic analysis that includes both species of Siamotherium confirms their basal position within the Hippopotamoidea.