TET2 DNA dioxygenase is frequently mutated in human hematopoietic malignancies and functionally inactivated in many solid tumors through a nonmutational mechanism. We recently found that TET2 mediates the interferon-JAK-STAT pathway to stimulate chemokine expression and tumor infiltration of lymphocytes (TILs). TET2 is monoubiquitylated at K1299, which promotes its activity. Here, we report that USP15 is a TET2 deubiquitinase and inhibitor. USP15 catalyzes the removal of K1299-linked monoubiquitin and negatively regulates TET2 activity. Gene expression profiling demonstrates that TET2 and USP15 oppositely regulate genes involved in multiple inflammatory pathways, and TET2 is a major target of USP15 function. Deletion of Usp15 in melanoma stimulates chemokine expression and TILs in a TET2-dependent manner, leading to increased response to immunotherapy and extended life span of tumor-bearing mice. These results reveal a previously unknown regulator of TET2 activity and suggest USP15 as a potential therapeutic target for immunotherapy of solid tumors.