Purpose: To determine the prevalence and associations of pterygium in a multiethnic adult population in rural China and to examine potential ethnic differences Methods: A total of 6418 adults (2133 ethnic Bai, 2130 ethnic Yi, and 2155 ethnic Han) aged 50 years or older participated in the study. Anterior segment examination was performed without pupil dilation using a slit lamp. Pterygium was de?ned as a raised fleshy triangular ?brovascular tissue growth of the conjunctiva encroaching onto a clear cornea. Results: Pterygium was least prevalent among adults of Yi ethnicity (29.5%) compared with Bai (39.0%, p < 0.001) or Han (39.5%, p < 0.001) ethnicities. Those of Bai ethnicity were most likely to be affected by severe pterygium (7.5%), while Yi were least likely to be affected (3.8%). Multivariate analysis revealed that increasing age (odds ratio, OR, 1.017), female sex (OR 1.53), lower educational level (OR 1.25), higher blood pressure levels (OR 1.002), and greater time spent outdoors per day (OR 1.09) were significantly associated with a higher prevalence of pterygium, while Yi ethnicity (OR 0.65, comparing Yi with Han) and wearing brimmed hats outdoors (OR 0.82) were protective factors. Conclusion: Ethnicity was significantly associated with prevalence of pterygium. Our findings may be applicable to many other countries located within the "pterygium belt" for health resource allocation among different ethnic groups.