Abstract Background The present study aimed to explore the threshold-effect association of serum ferritin levels with type 2 diabetes mellitus (T2DM) and prediabetes mellitus in Chinese adults. Methods A total of 8365 people from CHNS a cross-sectional survey in 2009 were finally included. The biomarker data, including major cardiovascular biomarkers and important nutrition biomarkers were collected. The association of serum ferritin levels with T2DM and prediabetes mellitus were assessed by using restricted cubic spline function combined with multivariate logistic regression model. Results The mean age of the study subjects was 50.3 years, and 46.5% were men. The risk of T2DM and prediabetes mellitus increased when the ferritin level was greater than 140 ng/ml. The OR(OR = 0.59, 95% CI, 0.35–0.98)was lowest between 40 to