Ewing sarcoma (ES) is an aggressive pediatric cancer that has remained refractory to current therapeutics. Immunotherapy has been unsuccessful in ES, largely due to poor understanding of how its immune tumor microenvironment (TME) is regulated. We recently demonstrated that ubiquitin-specific protease 6 (USP6) can remodel the ES immune landscape to engender an anti-tumorigenic TME. USP6 expression in ES cells enhances surface expression of immunostimulatory ligands and receptors, and induces production of multiple chemokines, driving recruitment and activation of tumor-suppressive immune lineages, including natural killer (NK) cells. We sought to harness this multi-faceted immunostimulatory function into a novel therapeutic by delivering in vitro transcribed USP6 mRNA via ionizable lipid nanoparticles (LNPs). Treatment of ES cells with USP6 mRNA in vitro is capable of inducing the aforementioned anti-tumorigenic and immunostimulatory responses. In addition, USP6 mRNA-treated ES cells elicit cytolytic activation of primary human CD8+ and CD4+ T lymphocytes and NK cells in vitro. Intratumoral (IT) delivery of USP6 mRNA LNPs suppresses growth of ES xenografts, coincident with increased immune infiltration and activation. We further demonstrate that USP6 mRNA is capable of igniting an immunostimulatory program in other cancer types (including acute myeloid leukemia (AML), melanoma, prostate cancer, head and neck cancer, and osteosarcoma) in vitro, and suppressing AML xenograft growth in vivo. Treatment with USP6 mRNA LNPs was well-tolerated, with no observed gross toxicity. Together, these pre-clinical studies provide proof-of-concept for the immunogenic and anti-tumorigenic efficacy of USP6 mRNA LNPs, and support its promise as a novel immunotherapeutic in diverse cancer types.