NKp46⁺ innate lymphoid cells (ILCs) serve important roles in regulating the intestinal microbiota and defense against pathogens. Whether NKp46⁺ ILCs arise directly from lymphoid tissue–inducer (LTi) cells or represent a separate lineage remains controversial. We report here that the transcription factor T-bet (encoded by Tbx21) was essential for the development of NKp46⁺ ILCs but not of LTi cells or nuocytes. Deficiency in interleukin 22 (IL-22)-producing NKp46⁺ ILCs resulted in greater susceptibility of Tbx21^−/− mice to intestinal infection. Haploinsufficient T-bet expression resulted in lower expression of the signaling molecule Notch, and Notch signaling was necessary for the transition of LTi cells into NKp46⁺ ILCs. Furthermore, NKp46⁺ ILCs differentiated solely from the CD4⁻ LTi population, not the CD4⁺ LTi population. Our results pinpoint the regulation of Notch signaling by T-bet as a distinct molecular pathway that guides the development of NKp46⁺ ILCs.