We investigated the thermoresponsive behavior of poly­(acrylamide-co-acrylonitrile) (poly­(AAm-co-AN)) with an upper critical solution temperature (UCST) in water to apply this polymer in cosmetic products that can be easily removed by washing with hot water. Six kinds of poly­(AAm-co-AN) polymers were synthesized, and those containing higher contents of AN units showed higher UCSTs in water. The amount of AN units in the resulting copolymer was controlled only by adding AN immediately prior to the polymerization to prevent its loss due to volatilization. We also analyzed the UCST behavior of poly­(AAm-co-AN) by fluorometry by labeling the polymer with an environment-sensitive fluorophore. The fluorescence spectra, fluorescence quenching titration results, and time-resolved fluorescence anisotropy measurements revealed that poly­(AAm-co-AN) loosely aggregated below the UCST, and the dipole–dipole interaction between the cyano groups in AN units and the hydrogen bonds between the amide groups in the AAm units were responsible for the cross-linking of the aggregates.