Poly(sodium styrenesulfonate)-block-poly(acrylic acid) (PNaSS-b-PAA) and poly(sodium styrenesulfonate)-block-poly(N-isopropylacrylamide) (PNaSS-b-PNIPAM) were prepared via reversible addition-fragmentation chain transfer (RAFT) radical polymerization using a PNaSS-based macro-chain transfer agent. The molecular weight distributions (M w /M n ) of PNaSS-b-PAA and PNaSS-b-PNIPAM were 1.18 and 1.39, respectively, suggesting that these polymers have controlled structures. When aqueous solutions of PNaSS-b-PAA and PNaSS-b-PNIPAM were mixed under acidic conditions, water-soluble PNaSS-b-PAA/PNaSS-b-PNIPAM complexes were formed as a result of hydrogen bonding interactions between the pendant carboxylic acids in the PAA block and the pendant amide groups in the PNIPAM block. The complex was characterized by 1 H NMR, dynamic light scattering, static light scattering, and transmission electron microscope measurements. The light scattering intensity of the complex depended on the mixing ratio of PNaSS-b-PAA and PNaSS-b-PNIPAM. When the molar ratio of the N-isopropylacrylamide (NIPAM) and acrylic acid (AA) units was near unity, the light scattering intensity reached a maximum, indicating stoichiometric complex formation. The complex dissociated at a pH higher than 4.0 because the hydrogen bonding interactions disappeared due to deprotonation of the pendant carboxylic acids in the PAA block.