Bladder cancer is lethal in its advanced, muscle-invasive phase with very limited therapeutic advances^,. Recent molecular characterization has defined new (epi)genetic drivers and potential targets for bladder cancer^,. The immune checkpoint inhibitors have shown remarkable efficacy but only in a limited fraction of bladder cancer patients^{, , –}. Here, we show that high G9a (EHMT2) expression is associated with poor clinical outcome in bladder cancer and that targeting G9a/DNMT methyltransferase activity with a novel inhibitor (CM-272) induces apoptosis and immunogenic cell death. Using an immunocompetent quadruple-knockout (Pten^loxP/loxP; Trp53^loxP/loxP; Rb1^loxP/loxP; Rbl1^−/−) transgenic mouse model of aggressive metastatic, muscle-invasive bladder cancer, we demonstrate that CM-272 + cisplatin treatment results in statistically significant regression of established tumors and metastases. The antitumor effect is significantly improved when CM-272 is combined with anti-programmed cell death ligand 1, even in the absence of cisplatin. These effects are associated with an endogenous antitumor immune response and immunogenic cell death with the conversion of a cold immune tumor into a hot tumor. Finally, increased G9a expression was associated with resistance to programmed cell death protein 1 inhibition in a cohort of patients with bladder cancer. In summary, these findings support new and promising opportunities for the treatment of bladder cancer using a combination of epigenetic inhibitors and immune checkpoint blockade.