An aqueous polyurethane resin having a cationic group in a side chain thereof is obtainable by reacting a polyester polyol, a polyisocyanate, and a diol containing a quaternary N-atom or amino group, the quaternary N-atom or amino group being not present in the carbon chain between the two hydroxyl groups of the diol. The polyester polyol is obtained by reacting an aromatic polycarboxylic acid and a polyol. The polyurethane resin is suitable as a resin in treatment liquids for inkjet printing.