Retroviral gene transfer of interleukin-12 (IL-12) into T cells markedly enhances antitumor efficacy upon adoptive transfer but has clinically shown unacceptable severe side effects. To overcome the toxicity, we engineered tumor-specific CD8+ T cells to transiently express IL-12. Engineered T cells injected intratumorally, but not intravenously, led to complete rejections not only of the injected lesion but also of distant concomitant tumors. Efficacy was further enhanced by co-injection with agonist anti-CD137 mAb or by transient co-expression of CD137 ligand. This treatment induced epitope spreading of the endogenous CD8+ T cell immune response in a manner dependent on cDC1 dendritic cells. Mouse and human tumor-infiltrating T lymphocyte cultures can be transiently IL-12 engineered to attain marked immunotherapeutic effects. Display omitted •mRNA electroporation engineers T cells to transiently produce single-chain IL-12•IL-12-engineered T cells injected intratumorally reject local and distant tumors•Co-injection of anti-CD137 mAb or co-electroporation of CD137L enhances efficacy•IL-12-engineered TIL cultures successfully treat autologous engrafted tumors Etxeberria et al. engineer tumor-specific CD8+ T cells that transiently express IL-12 to mitigate toxicity and show that intratumoral injection of these engineered T cells in conjunction with local CD137 co-stimulation leads to epitope spreading and regression of injected and distant lesions in solid tumor models.