STAT4 has been consistently associated with several autoimmune diseases, including rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). The aim of this study was to determine whether the STAT4 rs7574865G/T polymorphism confers susceptibility for RA and SLE in a sample of Mexican patients. This study included 869 individuals: 415 patients with RA, 128 patients with SLE, and 326 controls. Genotyping using TaqMan probes showed an association between the STAT4 rs7574865G/T polymorphism and RA (GG vs. TT: OR 1.99, p=0.0009; G vs. T: OR 1.42, p=0.0009) and SLE (GG vs. TT: OR 2.98, 0.0003; G vs. T: OR 1.74, p=0.0002). Gender stratification showed an association with RA (GG vs. TT: OR 1.99, 95% CI 1.3-3.1, p=0.002; G vs. T: OR 1.42, 95% CI 1.1-1.8, p=0.002) and SLE (GG vs. TT: OR 3.3, 95% CI 1.7-6.2, p=0.0002; G vs. T: OR 1.8, 95% CI 1.3-2.4, p=0.0002) in women. Thus, the STAT4 rs7574865G/T polymorphism confers risk for RA and SLE in the Mexican population.