It is a challenge to invigorate tumor-infiltrating lymphocytes without causing immune-related adverse events, which also stands as a primary factor contributing to resistance against cancer immunotherapies. Interleukin (IL)-15 can potently promote expansion and activation of T cells, but its clinical use has been limited by dose-limiting toxicities. In this study, we develop a tumor-conditional IL-15 (pro-IL-15), which masks IL-15 with steric hindrance caused by Fc fragment and IL-15Rα-sushi domain. Upon reaching the tumor site, it can be cleaved by tumor-associated proteases to release an IL-15 superagonist, resulting in potent antitumor activities. Systemic delivery of pro-IL-15 demonstrates significantly reduced toxicity but uncompromised antitumor efficacy. Pro-IL-15 can yield better effectors and vitalize terminally exhausted CD8+ T cells to overcome checkpoint blockade resistance. Moreover, pro-IL-15 promotes chemotaxis and activation of adoptive T cells, leading to eradication of advanced solid tumors and durable cures. Furthermore, pro-IL-15 shows promise for synergizing with other immunotherapies like IL-12 and oncolytic virus by improving the CD8/Treg ratio and interferon-γ levels, resulting in substantial regression of both local and metastatic cold tumors. Collectively, our results suggest that pro-IL-15 represents a compelling strategy for overcoming resistance to current immunotherapies while avoiding toxicities.Copyright © 2024 The American Society of Gene and Cell Therapy. Published by Elsevier Inc. All rights reserved.