Thymic stromal lymphopoietin (TSLP) is an IL-7-like type 1 inflammatory cytokine that is mainly produced by epithelial cells in the skin, lungs, thymus, and gastrointestinal tract. This cytokine is a master regulator involved in T helper 2 cell-type inflammation immune responses. Various cell types, including T, B, mast, dendritic, and cancer or cancer-associated cells, are activated via TSLP. TSLP expression is also associated with various human cancers and produced by Helicobacter pylori-infected human gastric epithelial cells. TSLP is a multi-functional protein that can act as both an oncogene and a tumor suppressor. The aim of this study was to examine the role of TSLP in the progression of gastric cancer (GC) and its correlation with clinicopathological features in GC patients. Because of the relationship between H. p ylori infection and GC, we also examined gastric tissue specimens for H. p ylori DNA. In this study, fresh tumoral tissues and distant tumor-free samples from 50 GC patients were assessed for TSLP mRNA expression by quantitative real-time PCR. The GC samples were also assessed for H. p ylori DNA using primers specific for H. p ylori 16S rRNA and the UreC genes by PCR. TSLP mRNA was overexpressed in 20 of the 50 (40%) GC samples relative to their corresponding normal tissues. TSLP overexpression was significantly correlated with tumor cell metastasis to lymph nodes. Of the 20 patients with TSLP overexpression, 17 (85.0%) had metastasis to lymph nodes (p = 0.023). In addition, the presence of H. p ylori was confirmed by PCR in 22 of the 50 (44%) cases and 10 (50%) of the 20 TSLP overexpressors. We show that human GC cells produce TSLP and a significant correlation was seen between TSLP overexpression and GC metastasis to lymph nodes. This is the first report to indicate that TSLP may play a role in lymph node involvement in GC patients.