The TNF-like cytokine TL1A augments IFN-γ production by anti-CD3 plus anti-CD28 and IL-12/IL-18-stimulated peripheral blood (PB) T cells. However, only a small subset of PB T cells respond to TL1A stimulation with IFN-γ production. PB CCR9+ T cells represent a small subset of circulating T cells with mucosal T cell characteristics and a Th1/Tr1 cytokine profile. In the current study, we show that TL1A enhanced IFN-γ production by TCR-or CD2/CD28-stimulated CCR9+ CD4+ PB T cells. However, TL1A had the most pronounced effect on augmenting IFN-γ production by IL-12/IL-18-primed CCR9+ CD4+ PB T cells. TL1A enhanced both the percentage and the mean fluorescence intensity of IFN-γ in CCR9+ CD4+ T cells as assessed by intracellular cytokine staining. IL-12 plus IL-18 up-regulated DR3 expression in CCR9+ CD4+ T cells but had negligible effect on CCR9− CD4+ T cells. CCR9+ CD4+ T cells isolated from the small intestine showed a 37-to 105-fold enhancement of IFN-γ production when TL1A was added to the IL-12/IL18 cytokine combination. Cell membrane-expressed TL1A was preferentially expressed in CCR9+ CD4+ PB T cells, and a blocking anti-TL1A mAb inhibited IFN-γ production by cytokine-primed CCR9+ CD4+ T cells by∼ 50%. Our data show that the TL1A/DR3 pathway plays a dominant role in the ultimate level of cytokine-induced IFN-γ production by CCR9+ mucosal and gut-homing PB T cells and could play an important role in Th1-mediated intestinal diseases, such as Crohn’s disease, where increased expression of IL-12, IL-18, TL1A, and DR3 converge in the inflamed intestinal mucosa.