A significant and reversible size change (up to 80 nm, representing 100 % increase in diameter, i. e. “breathing”) of polymeric micelles of poly(styrene)‐block‐poly(3‐(4‐vinylpyridin‐1‐ium‐1‐yl)propane‐1‐sulfonate) (PS‐P4VP‐zwitter) was observed during phototriggered loading and release of spiropyran. The breathing (i. e. “inhaling/exhaling” of the small molecules by the micelles) could be controlled by switching the wavelength of the light used. Spiropyran, which was converted into the zwitterionic merocyanine by UV light, was readily diffused and incorporated into the zwitterionic micelle cores, thus causing the micelles to expand. At the same time, the merocyanine was stabilized by the polyzwitterionic core of the micelles. To the best of our knowledge, this is the first report of a supramolecular and reversibly photoloadable system that can be converted from a thermodynamically stable self‐assembled equilibrium system into a dynamic, self‐assembled system that is sustained by illumination.