In this contribution, wave localization in a disordered periodic viaduct (DPV) undergoing out-of-plane vibration is investigated. The DPV is assumed to be composed of infinite spans with each span deviating from the standard span slightly. Each span is supposed to be composed of two longitudinal beams and a pier linked by three springs. By using the governing equations for the pier and beams as well as the joint conditions at the beam-beam-pier junction, the transfer matrix for each span of the viaduct undergoing out-of-plane vibration is derived. Based on the derived transfer matrix for each span, the wave transfer matrices for the spans of the DPV are obtained. According to the Wolf’s algorithm, the Lyapunov exponents for the wave motion of the DPV are calculated. With the proposed model, the influences of the pier-height and beam-length disorders on the wave localization are examined. Also, the interactive effect of the damping and disorders on the wave localization in the DPV is investigated. Moreover, by the wave transfer matrix method, the wave conversion phenomenon in the DPV is studied.