Purpose <p>Therapeutic options targeting programmed cell death protein 1 (PD-1) and programmed death ligand-1 (PD-L1) have been approved for use in human malignancies, showing clinical benefits. Nonetheless, a significant number of patients, particularly those with prostate cancer (PCa), show poor response to anti-PD-1/PD-L1 therapies, highlighting the necessity to explore supportive strategies that could enhance conventional PD-1/PD-L1-targeting immunotherapy.</p> Methods <p>PCa cell lines DU145 and RM1 were used to investigate the effects of pirenzepine (PZP) on the proliferation, colony formation, and migration of PCa cells in vitro. A subcutaneous tumor model bearing DU145 and RM1 was established to evaluate the anti-tumor effect in vivo. Lentivirus transfection and quantitative polymerase chain reaction (qPCR) assays were conducted to investigate the role of STING (endoplasmic reticulum-associated protein stimulator of interferon genes) in the antitumor mechanisms of PZP. The infiltration of CD8⁺ T cells in tumor tissues was examined using immunohistochemistry. The RM1 subcutaneous tumor model was employed to assess the combined effects of PZP and anti-programmed cell death 1 antibody (anti-PD1) immune checkpoint blockade therapy.</p> Results <p>The in vitro and in vivo experiment results indicated that PZP suppressed the proliferation, colony formation, and migration of PCa cells in a dose-dependent manner. Notably, PZP also promoted CD8⁺ T cell infiltration and enhanced the anti-PD1 therapeutic effect on PCa. Mechanistically, the results preliminarily indicated that PZP impeded PCa progression and stimulated tumor immune response by upregulating STING expression.</p> Conclusions <p>Our results revealed that PZP impaired the malignant biological behavior of PCa and enhanced antitumor immunity. These findings may provide a theoretical basis for combining PZP with anti-PD1 immune checkpoint blockade therapy on PCa.</p>