Plant mannanases are enzymes that carry out fundamentally important functions in cell wall metabolism during plant growth and development by digesting manno-polysaccharides. In this work, the Arabidopsis mannanase 5-2 (AtMan5-2) from a previously uncharacterized subclade of glycoside hydrolase family 5 subfamily 7 (GH5_7) has been heterologously produced in Pichia pastoris. Purified recombinant AtMan5-2 is a glycosylated protein with an apparent molecular mass of 50 kDa, a pH optimum of 5.5-6.0 and a temperature optimum of 25 degrees C. The enzyme exhibits high substrate affinity and catalytic efficiency on mannan substrates with main chains containing both glucose and mannose units such as konjac glucomannan and spruce galactoglucomannan. Product analysis of manno-oligosaccharide hydrolysis shows that AtMan5-2 requires at least six substrate-binding subsites. No transglycosylation activity for the recombinant enzyme was detected in the present study. Our results demonstrate diversification of catalytic function among members in the Arabidopsis GH5_7 subfamily.