A better understanding of the mechanisms regulating CD8+ T cell differentiation is essential to develop new strategies to fight infections and cancer. Using genetic mouse models and blocking antibodies, we uncovered cellular and molecular mechanisms by which Notch signaling favors the efficient generation of effector CD8+ T cells. Fibroblastic reticular cells from secondary lymphoid organs, but not dendritic cells, were the dominant source of Notch signals in T cells via Delta-like1/4 ligands within the first 3 days of immune responses to vaccination or infection. Using transcriptional and epigenetic studies, we identified a unique Notch-driven T cell-specific signature. Early Notch signals were associated with chromatin opening in regions occupied by bZIP transcription factors, specifically BATF, known to be important for CD8+ T cell differentiation. Overall, we show that fibroblastic reticular cell niches control the ultimate molecular and functional fate of CD8+ T cells after vaccination or infection through the delivery of early Notch signals.