The transmembrane protein Tim-3 has received significant attention in recent years as a possible immunotherapy target. This is due to its robust expression on dysfunctional, exhausted, T cells found in the settings of cancer and chronic infection and biochemical evidence suggesting an inhibitory function of Tim-3. However, numerous clinical trials of putative Tim-3 blocking antibodies have yielded modest benefits, at best, in clinical trials for various cancers. Thus, there is a need to more fully understand the function of Tim-3 . Here we have studied the function of Tim-3 early during a T cell response to LCMV Armstrong, which causes an acute viral infection in mice. We show that Tim-3 is rapidly expressed after infection and that the expression of Tim-3 is associated with acquisition of a type I effector phenotype, including expression of T-bet and downregulation of Tcf-1, by both CD4+ and CD8+ T cells. In addition, we demonstrate that knockout or cytoplasmic truncation of Tim-3 attenuates the acquisition of the effector program by T cells after LCMV Armstrong infection. Together, these data help to clarify the role of Tim-3 during acute infection.