<p>During chronic infection and tumor progression, CD8⁺ T cells lose their effector functions and become exhausted. These exhausted CD8⁺ T cells are heterogeneous and comprised of progenitors that give rise to effector-like or terminally-exhausted cells. The precise cues and mechanisms directing subset formation are incompletely understood. Here, we show that growth factor independent-1 (Gfi1) is dynamically regulated in exhausted CD8⁺ T cells. During chronic LCMV Clone 13 infection, a previously under-described Ly108⁺CX₃CR1⁺ subset expresses low levels of Gfi1 while other established subsets have high expression. Ly108⁺CX₃CR1⁺ cells possess distinct chromatin profiles and represent a transitory subset that develops to effector-like and terminally-exhausted cells, a process dependent on Gfi1. Similarly, Gfi1 in tumor-infiltrating CD8⁺ T cells is required for the formation of terminally differentiated cells and endogenous as well as anti-CTLA-induced anti-tumor responses. Taken together, Gfi1 is a key regulator of the subset formation of exhausted CD8⁺ T cells.</p>