Background: Single-nucleotide polymorphism (SNP) rs2476601 within protein tyrosine phosphatase non-receptor type 22 gene (PTPN22) has been shown to be a risk factor for different autoimmune diseases. This study explored the association of 1858 C/T SNP with rheumatoid arthritis (RA) and celiac disease (CD) in a region covering south-west of Iran. Methods: Totally, 52 patients with CD, 120 patients with RA and 120 healthy subjects were selected. The samples were genotyped for the rs2476601 in PTPN22 gene using the tetra-amplification refractory mutation system polymerase chain reaction. Results: The frequency of +1858T risk allele was significantly increased in both RA (P=0.021, OR=2.56, 95%CI=1.19-5.47) and CD (P=0.002, OR=3.87, 95%CI=1.68-8.95) patients, as compared to the control group. However, no association was found between the +1858C/T PTPN22 gene SNP and the anti-cyclic citrullinated peptide and rheumatoid factor positivity in RA patients. Conclusions: PTPN22 gene could play a crucial role in people’s susceptibility to some autoimmune diseases.