Provided is an ABA block copolymer that can offer a pigment dispersion composition having excellent dispersion stability when used as a pigment dispersant in an aqueous medium. An ABA block copolymer including: an A block containing a structural unit represented by a general formula (1) below and a structural unit derived from a vinyl monomer with an acid group; and a B block containing a structural unit derived from a vinyl monomer with an aromatic ring group or an alicyclic alkyl group, the ABA block copolymer having an acid value of 30 to 250 mgKOH/g. In the general formula (1), R1 is a hydrogen atom or a methyl group and R2 is an alkyl group having 2 to 10 carbon atoms.