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Summary: Group 2 innate lymphoid cells (ILC2s) were detected in the peripheral blood and the joints of rheumatoid arthritis (RA) patients, serum-induced arthritis (SIA), and collagen-induced arthritis (CIA) using flow cytometry. Circulating ILC2s were significantly increased in RA patients compared with healthy controls and inversely correlated with disease activity. Induction of arthritis in mice led to a fast increase in ILC2 number. To elucidate the role of ILC2 in arthritis, loss- and gain-of-function mouse models for ILC2 were subjected to arthritis. Reduction of ILC2 numbers in RORαcre/GATA3fl/fl and Tie2cre/RORαfl/fl mice significantly exacerbated arthritis. Increasing ILC2 numbers in mice by IL-25/IL-33 mini-circles or IL-2/IL-2 antibody complex and the adoptive transfer of wild-type (WT) ILC2s significantly attenuated arthritis by affecting the initiation phase. In addition, adoptive transfer of IL-4/13-competent WT but not IL-4/13−/− ILC2s and decreased cytokine secretion by macrophages. These data show that ILC2s have immune-regulatory functions in arthritis. : The role of ILC2s in the initiation phase of inflammatory arthritis has remained unclear. Omata et al. demonstrate that the ILC2-derived IL-4/13 decrease pro-inflammatory cytokine secretion by macrophages, thereby attenuating arthritis. Keywords: innate lymphoid cells, rheumatoid arthritis, immunoregulation