Photoinduced changes in the spin wave spectrum of annealed pure YIG thin films have been measured at 23.3 GHz and positively identified as arising from a photoinduced magnetic surface anisotropy field. The effect depends upon the thermal history of the YIG thin film sample. For example, after cooling 20 K, illumination with white light (1.2 joules/cm2 or greater) reduces the resonance field separation of a surface spin wave mode and nearest body spin wave mode by 50% while the body mode incurs negligible shift. At 20 K, a photoinduced magnetic surface energy .0027 erg/cm2 is necessary to account and other measurements sensitive to the SWR boundary conditions. The measured photoinduced surface energy is an order of magnitude smaller at 175 K.