AbstractInterleukin‐38 (IL‐38) is a novel member of the IL‐1 cytokine family with anti‐inflammatory activity. However, its effect on adipogenesis and inflammatory cytokines secretion of adipocytes in vitro has not been reported. To address whether IL‐38 inhibits adipogenesis and inflammation in vitro, adipose precursor 3T3‐L1 cells were cultured with or without IL‐38. The morphology and size of lipid droplets in 3T3‐L1 cells were measured by Oil red O staining. The mRNA expression levels of GATA‐binding protein‐3 (GATA‐3), glucose transporter type 4 (GLUT4), peroxisome proliferator‐associated receptor γ2, IL‐1β, IL‐6, and monocyte chemoattractant protein‐1 (MCP‐1) in 3T3‐L1 cells were detected by real‐time PCR, The contents of IL‐6, IL‐1β, and MCP‐1 in 3T3‐L1 cell medium supernatants were determined by enzyme‐linked immunosorbent assay. IL‐38 significantly decreased the number of lipid droplets in 3T3‐L1 cells. IL‐38 also increased GATA‐3 and GLUT4 mRNA expression and inhibited IL‐1β, IL‐6, and MCP‐1 secretion by 3T3‐L1 cells. It is concluded that IL‐38 can inhibit the differentiation of human adipocytes and inflammatory cytokine production by 3T3‐L1 cells.