The detailed dental anatomy of sauropod mamenchisaurids remains largely unexplored. Here, we describe a well-preserved isolated sauropod tooth from the Upper Jurassic Qigu Formation of the Turpan-Hami Basin, Xinjiang, using high-resolution micro-CT and three-dimensional reconstruction to investigate its internal anatomy. This tooth exhibits a unique combination of features of Mamenchisauridae, specifically the presence of marginal denticles restricted to the mesial margin, and a circular distolingual boss. CT scan data provide novel insights into mamenchisaurid dental anatomy and present the first three-dimensional enamel distribution in sauropod teeth. The lingual ridge forms from thickened enamel and dentine, whereas the lingual boss arises solely from dentine expansion. A labiolingual enamel thickness asymmetry appears in the apical region, convergent with certain neosauropods. The pulp cavity shows a distinct volumetric transition, expanding basally into a bulbous root canal and appearing as a labiolingually compressed lamina structure in the crown. Taxonomic comparison indicates that this tooth represents a mamenchisaurid lineage distinct from the previously only known sauropod tooth from the Qigu Formation. Our study supports a diverse sauropod assemblage in the Qigu Formation and provides new anatomical evidence for understanding sauropod dental evolution.