Plenty of polyphenols, i.e. phenolic acids and flavonoids, were found in longan flower water extract (LFWE) through spectrophotometric and HPLC analyses. Antiobesity and hypolipidemic effects of polyphenol-rich longan flower water extract (LFWE) were investigated in this study. Eight male rats per group were assigned randomly to one of the following dietary groups: (1) normal-caloric diet and pure water (NCD + NDW); (2) hypercaloric diet and pure water (HCD + NDW); (3) HCD and 1.25% (w/v) LFWE (HCD + 1.25% LFWE); (4) HCD and 2.5% (w/v) LFWE (HCD + 2.5% LFWE) for 9 weeks. Body weight, size of epididymal fat, serum triglyceride level and atherogenic index, and hepatic lipids were decreased (p < 0.05) in HCD rats by drinking 2.5% LFWE which may result from downregulated (p < 0.05) pancreatic lipase activity, and sterol regulatory element binding protein-1c (SREBP-1c) and fatty acid synthase (FAS) gene expressions, as well as upregulated (p < 0.05) LDL receptor (LDLR) and peroxisome proliferator-activated-receptor-alpha (PPAR-alpha) gene expressions, and also increased (p < 0.05) fecal triglyceride excretions. Therefore, polyphenol-rich LFWE indeed characterizes antiobesity and hypolipidemic effects in vivo.