Natural killer (NK) cells are rapidly acting innate lymphocytes that protect against viral infections and cancer. Given their cytotoxic potential, many have attempted to repurpose NK cells as immunotherapeutics. To augment the design of NK cell technologies, our lab focuses on how transcription factors regulate NK cell maturation and reactivity. Previously, we demonstrated that the germline deletion of Ets1 results in a severe dysregulation of the NK cell compartment. Ets1, however, is expressed in several precursor populations, and it remains unclear whether its expression is required in mature cells. In mature adaptive lymphocytes, a growing body of literature has highlighted that Ets1 expression promotes type I fates and collaborates with transcription factors known to support mature NK (mNK) cell differentiation and cytotoxicity. Therefore, I hypothesized that Ets1 may regulate mNK cell terminal differentiation and function. To address this hypothesis, I generated mice with an NK cell-directed condition deletion of Ets1 (Ets1 cKO, Ncr1^Cre Ets1^F/F) that deletes in undifferentiated mNK cells. With the Ets1 cKO mice, I show that deleting Ets1 severely decreases the number of differentiated cytotoxic cells and significantly limits the defense against metastatic B16. Single-cell RNA sequencing supports a block in maturation and suggests that Ets1 stabilizes an NK cell-associated gene program while restricting T cell-related genes. To test the requirement of Ets1 in differentiated mNK cells, I generated mice harboring a tamoxifen-inducible deletion of Ets1 (TAM-KO, Ncr1^ERT2 Ets1^F/F) that effectively deletes in cytotoxic cells without inducing a developmental block. Unlike the Ets1 cKO mice, the TAM-KO mice do not show reduced NK cell numbers. However, deleting Ets1 in differentiated cells promotes an activation-associated phenotype and terminal maturation. Together, these data support a model whereby Ets1 stabilizes the NK cell fate in undifferentiated mNK cells and limits maturation and activation in differentiated, cytotoxic cells.