Improving effector activity of antigen-specific T cells is a major goal in cancer immunotherapy. Despite the identification of several effector T cell (T_EFF)-driving transcription factors (TFs), the transcriptional coordination of T_EFF biology remains poorly understood. We developed an in vivo T cell CRISPR screening platform and identified a key mechanism restraining T_EFF biology through the ETS family TF, Fli1. Genetic deletion of Fli1 enhanced T_EFF responses without compromising memory or exhaustion precursors. Fli1 restrained T_EFF lineage differentiation by binding to cis-regulatory elements of effector-associated genes. Loss of Fli1 increased chromatin accessibility at ETS:RUNX motifs, allowing more efficient Runx3-driven T_EFF biology. CD8⁺ T cells lacking Fli1 provided substantially better protection against multiple infections and tumors. These data indicate that Fli1 safeguards the developing CD8⁺ T cell transcriptional landscape from excessive ETS:RUNX-driven T_EFF cell differentiation. Moreover, genetic deletion of Fli1 improves T_EFF differentiation and protective immunity in infections and cancer.