The antidiabetic effect of the Citrus junos Tanaka (also known as yuja or yuzu) was examined. Ethanol extract of yuja peel (YPEE) significantly stimulated 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-D-glucose (2-NBDG) uptake in C2C12 myotubes. However, ethanol extract of yuja pulp (YpEE) and water extract of yuja peel (YPWE) or pulp (YpWE) did not stimulate glucose uptake. In addition, peroxisome proliferator-activated receptor gamma (PPAR-γ) and AMP-activated protein kinase (AMPK) activities were increased by YPEE in a dose-dependent manner. Pretreatment of AMPK inhibitor decreased the glucose uptake stimulated by YPEE in C2C12 myotubes. We confirmed the anti-diabetic effect of YPEE in mice fed a high fat-diet (HFD). Compared with control mice on a normal diet (ND), these mice showed increased body weight, liver fat, insulin resistance, triacylglycerol (TG), and total cholesterol content. Addition of 5% YPEE significantly reduced the weight gain and rise in liver fat content, serum triacylglycerol (TG), total cholesterol, and insulin resistance found in mice fed a high-fat diet (HFD). Moreover, YPEE reduced the secretion of HFD-induced adipocytokines such as leptin and resistin. YPEE also resulted in increased phosphorylation of AMPK in muscle tissues. These results suggest that ethanol extract of yuja peel exerts anti-diabetic effects via AMPK and PPAR-γ in both cell culture and mouse models.