Deficiencies in DNA repair and DNA degrading nucleases lead to accumulation of cytosolic DNA. cGAS is a critical DNA sensor for the detection of cytosolic DNA and subsequent activation of the STING signaling pathway. Here, we show that the cGAS-STING pathway was unresponsive to STING agonists and failed to induce type I interferon (IFN) expression in many tested human tumor cells including DU145 prostate cancer cells. Inhibition of IL-6 or the downstream JAK2/STAT3 signaling restored responsiveness to STING agonists in DU145 cells. STING activity in murine TRAMP-C2 prostate cancer cells was critical for tumor rejection and immune cell infiltration. Endogenous STING agonists including double-stranded DNA and RNA:DNA hybrids present in TRAMP-C2 cells contribute to tumor rejection, but tumor growth was further suppressed by administration of cGAMP. Intratumoral co-injections of IL-6 significantly reduced the anti-tumor effects of cGAMP. In summary, STING in tumor cells contributes to tumor rejection in prostate cancer cells, but its functions are frequently suppressed in tumor cells in part via JAK2 and STAT3 pathways.