The BLLJ_1391 protein from Bifidobacterium longum subsp. longum JCM1217, a cytosolic β-N-acetylglucosaminidase belonging to glycoside hydrolase family (GH) 20, hydrolyzed lacto-N-triose II (LNTri) as well as chitin oligosaccharides. Its reaction was found to follow a substrate-assisted mechanism with anomeric retention, which is common for GH 20 enzymes. Homologous enzymes are found in genomic sequences of B. longum subsp. infantis, Bifidobacterium bifidum, and Bifidobacteium breve, all of which are infant gut-associated species of Bifidobacterium. The distribution resembles that of 1,3-β-galactosyl-N-acetylhexosamine phosphorylase, suggesting that the enzyme plays a role in metabolism of human milk oligosaccharides by hydrolyzing LNTri generated via the cytosolic hydrolysis of lacto-N-tetraose (LNT) by LNT 1,3-β-galactosidase.