Th17 cells have emerged as an important mediator in inflammatory and autoimmune diseases. However, recent studies suggest a potential impact of Th17 cells on tumor. The current study was designed to investigate the possible involvement of Th17 cells in gastric cancer. Compared with healthy volunteers, patients with gastric cancer had a higher proportion of Th17 cells in peripheral blood. Notably, the increased prevalence of Th17 cells was associated with clinical stage. In addition, increased populations of Th17 cells were present in tumor-draining lymph nodes with advanced disease. Furthermore, the mRNA expression levels of Th17-related factors (IL-17, IL-23p19, and RORC) in tumor tissues and the serum concentrations of IL-17 and IL-23 cytokines were significantly increased in patients with advanced gastric cancer. The results indicate that Th17 cells may contribute to gastric cancer pathogenesis.