T-cell immunotherapy has demonstrated remarkable clinical outcomes in certain hematologic malignancies. However, efficacy in solid tumors has been suboptimal, partially due to the hostile tumor microenvironment comprised of immune inhibitory molecules. One such suppressive agent abundantly expressed in solid tumors is Fas ligand (FasL), which can trigger apoptosis of Fas-expressing effector cells like T cells and natural killer (NK) cells. To alleviate this FasL-induced suppression of tumor-specific immune cells in solid tumors, we describe here the development of a Fas decoy that is secreted by engineered cells upon activation and sequesters the ligand, preventing it from engaging with Fas on the surface of effector cells. We further improved the immune-stimulatory effects of this approach by creating a Fas decoy and IL-15 cytokine fusion protein, which enhanced the persistence and antitumor activity of decoy-engineered as well as bystander chimeric-antigen receptor (CAR) T cells in xenograft models of pancreatic cancer. Our data indicate that secreted Fas decoys can augment the efficacy of both adoptively transferred and endogenous tumor-specific effector cells in FasL-expressing solid tumors.