The high spatiotemporal resolution of light as an external stimulus allows the control of shape, mechanical properties, and even forces generated by photoresponsive soft materials. For this purpose, supramolecular systems that respond readily and reversibly to photoirradiation and convert microscopic changes into macroscopic effects are needed. This work demonstrates the reversible light-responsive modulation of the osmotic pressure of an aqueous solution of an azobenzene-containing polymer (azopolymer) and α-cyclodextrin. Osmometry shows that this multivalent and photoresponsive host-guest complex can be used to modulate the concentration of solutes in the solution. Upon alternating irradiation with UV and blue light, the osmolality is reversibly switched by 28 mOsm kg-1. The switching amplitude increases linearly with the concentration of azopolymer and cyclodextrin. This drastic change in osmotic pressure is achieved by carefully designing an azopolymer that provides multivalent interactions as well as high water solubility. In this way, our study demonstrates a tunable control of colligative properties by photoinduced modulation of supramolecular interactions.