Reversible photoswitch of viscosity and photoresponsive binding of protein has been achieved in water solution of azobenzene-modified polyacrylate. We synthesized polymers with a few mol % hydrophobic side groups, including azobenzene groups with different spacers between the photochrome and the backbone. The binding of the protein (BSA) onto the polymers was investigated by capillary electrophoresis in dilute solution and theology (viscosity, dynamic moduli, and stress relaxation) in semidilute solution of polymer. In the dilute regime, BSA/polymer complexes are formed in equilibrium with unbound BSA. Both the length of the hydrophobic spacer on the azobenzene side group and the presence of additional n-alkyl side groups significantly affect the affinity of BSA for the polymer. In the semidilute regime, viscosity enhancement by several decades is obtained upon addition of BSA in polymer solutions and ascribed to physical cross-linking involving BSA. In the two regimes, light was shown to modify the binding properties. Reversible release of BSA (by up to 80% of the protein) was obtained by exposure to UV. Reversible viscosity swings by up to 40-fold were cycled for hours by alternative exposure to UV/vis light. Light-induced cis-trans isomerization of the azobenzene together with low concentration of photochrome in the samples made it possible to obtain rapid responses (half-time similar to 20 s) in solutions or in gels having thickness of the order of centimeters. An unprecedented degree of sensitivity is achieved thanks to the amplification provided by properties of optimal modified polymers. These properties are analyzed in term of the response of cross-links density, chain dynamics, and binding affinity.