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Objective. Innate lymphoid cells (ILCs) are a recently discovered group of cells that are essential to epithelial homeostasis and are implicated in psoriasis pathogenesis, yet they have never been reported in psoriatic arthritis (PsA). Methods. ILC classes and subsets were characterized in the peripheral blood (PB) of healthy controls, patients with psoriasis, and patients with PsA and in the synovial fluid (SF) of patients with PsA and patients with rheumatoid arthritis (RA). Cell surface marker expression and intracellular cytokine production following stimulation were analyzed using flow cytometry. Results. ILCs were identified in the SF and were 4-fold more abundant in PsA SF than in PsA PB. Fewer CCR61 ILCs were found in PsA PB than in healthy control PB, while PsA SF was enriched for CCR61 ILCs compared to PsA PB and RA SF. Natural cytotoxicity receptor NKp441 group 3 ILCs were rare in PB and RA SF, but abundant in PsA SF. Increased numbers of interleukin-17A (IL-17A)–producing ILCs were present in PsA SF compared to RA SF. CCR6, NKp44, and melanoma cell adhesion molecule (MCAM) were expressed on the cell surface of SF ILCs that produced IL-17A. The number of circulating NKp441, CCR61, and MCAM1 ILCs in blood was inversely correlated with PsA disease activity. Conclusion. Our findings indicate that PsA SF is enriched for group 3 ILCs that express CCR6 and NKp44, which distinguishes the synovial compartment from RA. The increased IL-17A production by SF ILCs indicates a novel role for ILCs in PsA. [ABSTRACT FROM AUTHOR]