Insulin resistance is a major cause of type 2 diabetes mellitus (T2DM). To determine the role of human resistin in T2DM, we analyzed single nucleotide polymorphisms (SNP) in the human resistin gene. We found that the G/G genotype of a resistin SNP at -420 in the promoter region was associated with T2DM (546 cases and 564 controls). Meta-analysis of 1,888 cases and 1,648 controls confirmed this association. Sp1 and Sp3 transcription factors specifically recognize 420G and enhance promoter activity in vitro. Resistin SNP-420 determines its monocyte mRNA and serum resistin levels. In 198 T2DM and 157 controls, fasting serum resistin levels were higher in subjects with T2DM than the control, and they were higher in subjects who carried -420G/G genotypes. Multiple regression analysis revealed that the SNP-420 genotype was the strongest determinant of serum resistin. The level of serum resistin is in the order of G/G, G/C and C/C genotypes, starting with the highest in the 2,078 community-dwelling Japanese subjects. Serum resistin level was correlated with insulin resistance, lower HDL cholesterol, and high-sensitivity C-reactive protein in the Japanese general population. Furthermore, serum resistin level was correlated with the number of microangiopathies and the accumulation of metabolic syndrome factor in T2DM. Together, the specific recognition of 420G by Sp1/3 increases human resistin promoter activity in monocytes, leading to enhanced serum resistin levels, thereby inducing insulin resistance, T2DM, and its complications.