Regioselective introduction of alpha-mannoside branches at C-6 of chitin and chitosan has been accomplished by a series of regioselective modification reactions starting from N-phthaloyl-chitosan as a key precursor. Glycosylation of the derived acceptor with reactive groups only at C-6 with an ortho ester of D-mannose proceeded smoothly in dichloromethane in the presence of trimethylsilyl trifluoromethanesulfonate, and the degree of branching was up to 0.6. Full deprotection gave chitosans with alpha-mannoside branches, which were subsequently transformed into the corresponding branched chitins by N-acetylation. The resulting branched polysaccharides showed a remarkable solubility in neutral water in sharp contrast to the insoluble linear chitin and chitosan. Concanavalin A exhibited a specific affinity for these products, which was ascribable to the presence of alpha-mannoside groups. Though nonnatural, the branched chitins were susceptible to lysozyme, and the enzymatic degradation was heavily dependent on the extent of branching. Furthermore, the branched chitosan exhibited considerable antimicrobial activity.