<p>Gastric cancer (GC) progression is linked to immune escape in the tumor microenvironment, yet the molecules regulating tumor-associated macrophage polarization and CD8⁺ T-cell exhaustion are unclear. This study analyzed TCGA data to examine SULF1 expression and its prognostic role. It used CRISPR/Cas9 and lentiviral methods in GC cells to test proliferation, invasion, and apoptosis, plus co-culture and flow cytometry to assess SULF1’s impact on macrophages and CD8⁺ T-cells. STAT3 signaling was studied via immunoblotting and nuclear translocation assays, and a mouse model tested SULF1’s therapeutic relevance. Results showed SULF1 was up-regulated in GC, tied to advanced stages and poor survival. SULF1 knockdown inhibited the malignant phenotypes of gastric cancer cells, including proliferation, migration, and invasion abilities, while promoting cell apoptosis; conversely, SULF1 overexpression enhanced these pro-tumor phenotypes. SULF1 activated macrophage STAT3, promoting M2 polarization and CD8⁺ T-cell dysfunction. In mice, SULF1 silencing reduced tumors and T-cell exhaustion, while supplementation reversed this. Conclusions: GC-secreted SULF1 creates an immunosuppressive microenvironment via STAT3-dependent pathways, and targeting SULF1–STAT3 may improve GC immunity.</p>