This article presents a wideband beam-steering antenna system for high-mobility millimeter-wave (mm-wave) systems. It can provide both continuous and discrete beam steering in two dimensions (elevation and azimuth) at a speed that is sufficient for various applications including some in defense. The antenna is completely passive and beam steering is achieved using near-field phase transformation by employing a pair of distinct rotatable stepped-dielectric phase transformers (SPTs) placed in the near-field region of a fixed radiating source. The antenna system has a steering and impedance-matching bandwidth of 40.6% from 26.5 to 40 GHz. A prototype of the beam-steering antenna system including a mechanical system to rotate each of the SPTs around the antenna axis has been fabricated and tested. The rotating SPT pair introduces a predetermined phase gradient to the input near-field and creates an output near-field that will radiate in an arbitrarily selected direction, which can be varied within a large conical region with a maximum apex angle of 104°. The system exhibits predicted and measured peak gains of 21.5 and 21.25 dBi, respectively, and the measured gain variation over 2-D beam steering is less than 3 dB except at 36 and 39 GHz, where it rises to 3.6 and 3.1 dB, respectively. This beam steering method obviates the need for expensive phase shifters and distribution networks, which are also lossy at mm-wave frequencies. The measured results validate the predicted wideband matching and steering performance of the system with close agreement.