The aim of this work was designed to investigate the antioxidant, antiaging, and hepatoprotective effects of alkali-extractable polysaccharides (AlAPS) and their three purified fractions (AlAPS-1, AlAPS-2, and AlAPS-3) from Agaricus bisporus in D-galactose induced aging mice. For in vitro antioxidant analysis, both AlAPS and its fractions exhibited moderate reducing power, Fe2+-chelating activities, and potent scavenging activities on hydroxyl and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals. The in vivo results demonstrated that the polysaccharides, especially AlAPS-2, showed potential antiaging and hepatoprotective effects by enhancing the antioxidant status, decreasing serum hepatic enzyme activities, and improving the lipid metabolism. This study suggested that the polysaccharides extracted and purified from A. bisporus could be exploited as a potent dietary supplement to attenuate aging and prevent age-related diseases.