The photoswitching behavior of polyethylene glycol-based hydrogels was determined by monitoring the changes in their swellability and absorption spectra upon exposure to alternating wavelengths of irradiation. Highly hydrophilic PEG-based hydrogels were prepared by the irradiation of a cinnamylidene acetate-terminated PEG solution (PEG-CA). The degree of swelling of the PEG-CA hydrogels was predictably modulated by alternating the wavelength of exposing irradiation. Exposing the PEG-CA gels to >300 nm irradiation resulted in the decrease of their swellability, while short exposure to 254 nm led to more hydrophilic gels. That is, the physical properties of the PEG-CA gels can be controlled by the selection of wavelength of irradiation. Interestingly, it was found that 254 nm irradiation could not only lead to the photoscission of the PEG-CA gels, but also initiate a crosslinking reaction between PEG-CA monomers. On the contrary, only photocrosslinking via a cyclobutane ring formation was observed under >300-nm irradiation. Finally, the factors that could affect an efficient photoswitching behavior of the PEG-CA gels were investigated.