A highly efficient β-1,4-mannanase-secreting strain, Pholiota adiposa SKU0714, was isolated and identified on the basis of its morphological features and sequence analysis of internal transcribed spacer rDNA. P. adiposa β-1,4-mannanase was purified to homogeneity from P. adiposa culture supernatants by one-step chromatography on a Sephacryl gel filtration column. P. adiposa β-1,4-mannanase showed the highest activity toward locust bean gum (V _max = 1,990 U/mg protein, K _m = 0.12 mg/mL) ever reported. Its internal amino acid sequence showed homology with hydrolases from the glycoside hydrolase family 5 (GH5), indicating that the enzyme is a member of the GH5 family. The saccharification of commercial mannanase and P. adiposa β-1,4-mannanase-pretreated rice straw by Celluclast 1.5L (Novozymes) was compared. In comparison with the commercial Novo Mannaway^® (113 mg/g-substrate), P. adiposa β-1,4-mannanase-pretreated rice straw released more reducing sugars (141 mg/g-substrate). These properties make P. adiposa β-1,4-mannanase a good candidate as a new commercial β-1,4-mannanase to improve biomass pretreatment.