Objective To explore the association between -23A/T variation of insulin gene (INS gene) and early-onset type 2 diabetes mellitus (T2DM). Methods A total of 116 Han patients with early-onset T2DM (the case group) and 135 non-diabetic controls (the control group) in Shanghai were enrolled. The -23A/T variation of INS gene was detected with PCR-direct sequencing. Differences in frequencies of genotypes and alleles of -23A/T, as well as other clinical variables between two groups were analyzed. Results  AA genotypic and A allelic frequencies significantly increased in the case group compared with the control group [P=0.015, OR=2.77 (95% CI 1.19-6.47); P=0.009, OR=2.86 (95% CI 1.26-6.46)]. In the case group, the fasting serum insulin (FINS) and HOMA-β levels of subjects carrying AA genotype were significantly lower than those of subjects carrying AT genotype (both P<0.05). Conclusion The A allele of -23A/T variation of INS gene is significantly associated with the FINS in patients with early-onset T2DM and is a risk factor for early-onset T2DM in Shanghai Han population. The INS gene-23A/T variation may be a potential genetic marker for predicting islet β-cell hypofunction in Chinese population with early-onset T2DM.