Metastable β-Ti alloys have recently gained popularity for the fabrication of medical implants such as prostheses. However, they suffer from poor tribological properties. Surface treatments such as plasma nitriding and ultrasonic shot peening both have been successfully used to this end. The ultrasonic shot peening treatment induces surface nanocrystallization and introduces many structural defects in the surface layers. These defects increase the reactivity of the surface and enhance the diffusion of chemical species, which is beneficial to the nitriding kinetics. In this study, an ultrasonic shot peening treatment was applied to two microstructures, namely equiaxial α + β and metastable β, of the Ti-5553 alloy before nitriding at low temperature. The results show much finer secondary α precipitation for the initial β microstructure subjected to USP and nitrided, the formation of sparsely dispersed equiaxial α grains, and that some of the deformation induced by the USP treatment remains after nitriding.