An ion implanter was used to irradiate protons with a dose of 8×1016 ions/cm2 and an energy of 400 keV into a Ce³⁺:GGG crystal. To our knowledge, this is the first time that ion implantation was applied to a Ce³⁺:GGG crystal. The penetration depth was determined by the stopping and range of ions in matter simulation. The prism coupling method was utilized to evaluate the propagation modes in the H⁺-implanted waveguide. The end-face coupling method and the finite-difference beam propagation were employed for the analysis of the waveguide performance. The refractive index distribution was reconstructed by the reflectivity calculation method. The main significance of the investigation is to confirm the potential of the ion-irradiated Ce³⁺:GGG crystals for fabricating waveguide structures and devices.