Agaricus bisporus is well known as a source of polysaccharides that could improve human health. The objective of this study was to explore anti-obesity effect of A. bisporus extract (H2Oslim®) abundant in polysaccharides and its underlying mechanism. Pancreatic lipase inhibitory activity in vitro was determined after treatment with H2Oslim® and chitosan. Treatment with H2Oslim® and chitosan significantly decreased pancreatic lipase activity. Five-week-old male SD rats were randomly divided into three groups for acute feeding with vehicle, H2Oslim® at 80 mg/kg body weight (BW)/day, and H2Oslim® at 160 mg/kg BW/day. H2Oslim® does-dependently increased plasma lipid clearance in an oral lipid tolerance test. Five-week-old male C57BL/6N mice were fed a control diet (CD), a high fat diet (HFD), an HFD with H2Oslim® at 80 mg/kg BW/day, H2Oslim® at 160 mg/kg BW/day, or chitosan at 160 mg/kg BW/day for eight weeks. HFD-fed mice showed significant increases of body weight, fat mass, white adipose tissue, average lipid droplet size, and serum levels of glucose, triglyceride, ALT, and AST compared to those in the CD group. However, H2Oslim® or chitosan administration ameliorated these increases. H2Oslim® or chitosan signifi-cantly reduced dietary efficiency and increased fecal excretion levels of lipid, triglyceride, and total cholesterol. These in vitro and in vivo findings suggest that H2Oslim® might act as an anti-obesity agent by inhibiting pancreatic lipase-mediated lipid absorption, at least in part.