Many gut functions are attuned to circadian rhythm. Intestinal group 3 innate lymphoid cells (ILC3s) include NKp46+ and NKp46- subsets, which are RORgt dependent and provide mucosal defense through secretion of interleukin-22 (IL-22) and IL-17. Because ILC3s highly express some key circadian clock genes, we investigated whether ILC3s are also attuned to circadian rhythm. We noted circadian oscillations in the expression of clock and cytokine genes, such as REV-ERBα, IL-22, and IL-17, whereas acute disruption of the circadian rhythm affected cytokine secretion by ILC3s. Because of prominent and rhythmic expression of REV-ERBα in ILC3s, we also investigated the impact of constitutive deletion of REV-ERBα, which has been previously shown to inhibit the expression of a RORgt repressor, NFIL3, while also directly antagonizing DNA binding of RORgt. Development of the NKp46+ ILC3 subset was markedly impaired, with reduced cell numbers, RORgt expression, and IL-22 production in REV-ERBα-deficient mice. The NKp46- ILC3 subsets developed normally, potentially due to compensatory expression of other clock genes, but IL-17 secretion paradoxically increased, probably because RORgt was not antagonized by REV-ERBa. We conclude that ILC3s are attuned to circadian rhythm, but clock regulator REV-ERBa also has circadian-independent impacts on ILC3 development and functions due to its roles in the regulation of RORgt.