A new species of the genus Aceratherium, A. porpani sp. nov., from the Tha Chang sand pits in Nakhon Ratchasima Province, northwestern Thailand, is described. It is a mid-sized rhinocerotid in the subfamily Aceratheriinae, and represents the first discovery of Aceratherium in Thailand. The material includes a well-preserved skull and mandible. A. porpani has broadly separated parietal crests, slightly expanded zygomatic arches, a straight nuchal crest, moderate supraorbital tuberosities, a flat skull roof, a deep nasal notch above the P4/M1 boundary, a moderately wide mandibular symphysis with a posterior border at the p3/p4 boundary, a short diastema between 12 and p2, absence of DP1 and dp1, strong crochets, constricted molar protocones, and long metalophs. This new species has a mixture of primitive and derived characters that differ from the known species of Aceratherium, A. incisivum, and A. depereti. The evolutionary stage of A. porpani is consistent with the latest Miocene age of the associated fauna and flora in the Tha Chang sand pits.