Adenosine deaminase (ADA) is an important regulator of insulin action. The single nucleotide polymorphism (SNP) G22A in the ADA gene decreases enzymatic activity of ADA. The aim of this study was to investigate the relationship between the SNP G22A and blood glycemic control, insulin resistance, and obesity of gestational diabetes mellitus (GDM) patients in an Iranian population. SNP G22A was determined in women with GDM (N=70) and healthy pregnant women (control, N=70) using polymerase chain reaction-restriction fragment length polymorphism. Fasting plasma glucose (FPG), hemoglobin A1C (HbA1c), plasma insulin levels and plasma lipids were measured using commercial kits. Homeostasis model of assessment for insulin resistance (HOMA-IR) was calculated. The distribution of genotypes and alleles among GDM patients was similar to that of the control group. FPG and HbA1c were significantly higher in GDM patients with GG genotype compared with GDM patients with GA+AA genotype and non-GDM patients. The frequency of GG genotype was significantly higher in obese GDM patients compared to lean GDM patients. The SNP G22A in the ADA gene was not associated with the risk of GDM in our population. GG genotype was associated with poor glycemic control and obesity in GDM patients.