A new hominoid fossil site, Chiang Muan in northern Thailand, yielded the first finding of a large-bodied Miocene hominoid in Southeast Asia. This specimen (CMu6-1′00) was preliminarily reported by Kunimatsu et al. ([2000a] Primate Res. 16:299). Later, Chaimanee et al. ([ 2003] Nature 422:61–65) reported additional hominoid teeth from the same site, but all of them were collected from younger deposits (the Upper Lignite Member, in Nagaoka and Suganuma [ 2002] Primate Res 18:159–164). The specimen described here (CMu6-1′00) was recovered from the Lower Lignite Member (Nagaoka and Suganuma [ 2002] Primate Res 18:159–164), which is probably several hundred thousand years older than the Upper Lignite Member (Suganuma et al. [ 2002] Primate Res. 18:165–173). This article provides a detailed description of this hominoid specimen and paleontological/geological data of the fossil site at Chiang Muan. The hominoid specimen (CMu6-1′00) is an isolated upper molar (right M1 or M2), similar in size to modern orangutans (Pongo pygmaeus). This upper molar has low and voluminous cusps, relatively thick enamel, and relatively low relief of the dentine/enamel junction, with only a faint remnant of the lingual cingulum. The age of Chiang Muan is estimated to be the latest Middle Miocene (ca. 11–12 Ma), based on the mammalian fossils (Nakaya et al. [ 2002] Primate Res. 18:131–141) and paleomagnetic study (Suganuma et al. [ 2002] Primate Res. 18:165–173). This suggests that the Chiang Muan Hominoid in the present study is an earlier member of Eastern Eurasian Miocene hominoids. Am J Phys Anthropol, 2003. © 2003 Wiley-Liss, Inc.