Background: Low serum vitamin D concentrations have been reported to be associated with an increased risk of non-alcoholic fatty liver disease (NAFLD). However, whether slim or obese people show a similar association between vitamin D and NAFLD remains speculative. This study aimed to explore the relationship between serum vitamin D concentrations and NAFLD in lean and obese Chinese adults.Methods: This cross-sectional study included 2538 participants (1360 men and 1178 women) who underwent their health checkups at the First Affiliated Hospital of Zhejiang University School of Medicine in 2019. NAFLD was diagnosed by liver ultrasound excluding other causes. The association of serum vitamin D concentrations with NAFLD was analyzed in lean and obese participants.Results: The overall prevalence of NAFLD was 33.61% (13.10% in lean and 53.32% in obese) in this study population. The serum vitamin D levels of obese NAFLD patients were lower than that of the obese NAFLD-free controls. However, the serum vitamin D levels of lean NAFLD patients were comparable to that of the lean NAFLD-free controls. Serum vitamin D levels were negatively correlated with the prevalence of NAFLD in obese but not lean participants. Serum vitamin D levels were independently associated with the risk of NAFLD in obese participants, with an adjusted odd ratio (95% CI) of 0.986 (0.979–0.992). However, the serum vitamin D levels were not related to the risk of NAFLD in lean participants.Conclusions: Low serum vitamin D levels are associated with NAFLD in obese but not lean participants.