Water chestnut (Trapa japonica) is a thorny aquatic floating plant that contains hydrolyzable polyphenols (WCPs) such as eugeniin, 1,2,3,6-tetra-O-galloyl-β-d-glucopyranose (TGG), and trapain. Previous studies have revealed that WCPs have antioxidant properties and can suppress postprandial blood glucose elevation in mice and human participants. However, the functionality of WCPs in treating obesity and metabolic syndrome, which have become significant issues in recent years, remains unclear. Here, we aimed to determine the effects of WCPs on lipid metabolism. We initially investigated the effects of WCPs on pancreatic lipase in vitro. Among WCPs, eugeniin and trapain, which contain a hexahydroxybiphenyl group, particularly inhibited lipase activity. We also found that WCPs suppressed fat accumulation in mouse adipose progenitor 3T3-L1 cells. We compared the effects of WCPs on lipid metabolism among groups of mice (n=7 each) fed for four weeks without (standard control and high-fat (HF) diet controls) or with HFWP 0.5 or HFWP 1 supplemented with 0.5 and 1% WCPs, respectively. Subsequently, we examined lipid-related parameters in adipose tissue, liver, and blood. We found significantly lower plasma triacylglycerol content in the HFWP1 than in the HF group. These findings suggest that WCPs may help prevent the onset of obesity and metabolic syndrome.