T cells within the tumor microenvironment frequently exhibit dysfunctional characteristics that compromise their ability to elicit both innate and therapeutic-induced immune responses. Regulators of immune dysfunction represent therapeutic targets to activate antitumor immunity. In this study, we identified semaphorin 3G (SEMA3G) as a key regulator of immune responses in cancer. SEMA3G was widely upregulated in diverse human cancers, and its expression was positively correlated with tumor progression. SEMA3G acted as a ligand that inhibited the activation and functionality of T cells. A comprehensive receptor screening approach demonstrated that SEMA3G exhibited a significantly stronger affinity for neuropilin (NRP) 1 than for NRP2. Furthermore, SEMA3G primarily impeded T-cell functions via NRP1. Disruption of SEMA3G using CRISPR/Cas9 technology or blockade with a neutralizing antibody effectively restored the cytotoxicity of CD8+ T cells and inhibited the growth of tumors in vivo. This research underscores the role of SEMA3G in T-cell dysfunction within tumors and proposes targeting SEMA3G as a cancer immunotherapeutic strategy. Significance: SEMA3G binding to NRP1 suppresses cytotoxic T-cell activity to induce an immunosuppressive tumor microenvironment, positioning SEMA3G as a promising therapeutic target for improving cancer immunotherapy.