Cancer immunotherapy is a potent treatment modality, but its clinical benefit depends on the tumor's immune profile. Here, we used mJX-594 (JX), a targeted and GM-CSF-armed oncolytic vaccinia virus, as a strategy to remodel the tumor microenvironment (TME) and subsequently increase sensitivity to αPD-1 and/or αCTLA-4 immunotherapy. The remodeling of the TME was determined using histologic, flow-cytometric, and NanoString immune profiling analyses. JX was intratumorally injected into implanted Renca kidney tumors or transgenic mouse breast cancers with or without αPD-1 and/or αCTLA-4. Various combination regimens were used to evaluate immunotherapeutic anticancer responses. Intratumoral injection of JX remodeled the TME through dynamic changes in the immune system, as shown by increased tumor-infiltrating T cells and upregulation of immune-related gene signatures. This remodeling induced conversion of a noninflamed tumor into an inflamed tumor. JX virotherapy led to enhanced abscopal effects in distant tumors, with increased intratumoral infiltration of CD8 T cells. A depletion study revealed that GM-CSF is an indispensable regulator of anticancer efficacy of JX. Dual-combination therapy with intratumoral JX and systemic αPD-1 or αCTLA-4 further enhanced the anticancer immune response, regardless of various treatment schedules. Of note, triple combination immunotherapy with JX, αPD-1, and αCTLA-4 elicited the most potent anticancer immunity and induced complete tumor regression and long-term overall survival. Our results show that intratumoral JX treatment induces dramatic remodeling of the TME and more potently suppresses cancer progression with immune-checkpoint blockades by overcoming resistance to immunotherapy.