Two-dimensional (2D) Synthetic Aperture Radar (SAR) and Inverse SAR (ISAR) imaging is of importance for diagnostic studies of target scattering mechanisms. Rail SAR and turntable ISAR are currently widely applied techniques for high resolution diagnostic imaging of complex targets. Three-dimensional (3D) imagery has the advantage of providing scattering magnitude and exact positions in altitude and in down-range and cross-range for each scattering center on a complex target. Thus, development of various 3D imaging radar systems for diagnostic measurements is attracting increasing attention from radar researchers. In this work, a novel 3D imaging method based on Multiple-Input Multiple-Output (MIMO) radar and interferometric SAR processing is proposed. First, a MIMO array capable of interferometric measurement with high aperture utilization ratio is designed and tested. Then, a signal model for interferometric MIMO radar is formulated. The relation between interferometric phase and scatterer altitude is specifically analyzed, and a 3D image formation algorithm is developed. Finally, a numerical simulation and field data acquired using experimental MIMO radar system are presented to demonstrate the feasibility and usefulness of the proposed method for target scattering diagnosis. The proposed method has more advantages than current 3D imaging techniques, such as high measurement efficiency, low research and development cost, and strong environmental adaptability.