Ultrahigh electromechanical coupling factors and piezoelectric strain coefficients of relaxor-based single crystals make them materials of choice for next-generation electroacoustic transducers. Thus far, the compositional homogeneity problem of relaxor-based single crystals has not been completely solved. To promote the application of these crystals, methods for evaluating their quality and uniformity using nondestructive processes are urgently needed. In this study, resonant ultrasound spectroscopy (RUS) was employed for the nondestructive evaluation (NDE) of the property inhomogeneity of relaxor-based single crystals for the first time. A inhomogeneous rectangular parallelepiped [0 0 1]c poled Mn-doped 0.23Pb(In1/2Nb1/2)O3–0.48Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 (23PIN-48PMN-29PT) single crystal sample and a homogeneous lead zirconate titanate (PZT-8) sample were evaluated via RUS. Furthermore, the evaluation results were verified by cutting the 23PIN-48PMN-29PT sample into four smaller samples and measuring the elastic constants , , and ; dielectric constants ; and piezoelectric constant d33 for each sample (including the PZT-8 sample).