Recently, there has been great interest in 3-D ultrasound imaging, but power constraints have precluded practical implementation of highresolution 3-D ultrasound in handheld imaging platforms. In this paper, we propose a separable beamforming procedure for subaperturebased 3-D synthetic aperture ultrasound systems that drastically reduces computational (and hence power) requirements. Separable beamforming approximates 2-D array beamforming through a series of 1-D beamforming operations. Our proposed method is based on a separable delay decomposition method that minimizes phase error. We show that the proposed separable system achieves 19- fold complexity reduction while producing images with contrastto- noise ratio comparable to that of non-separable systems. Furthermore, we propose an online iterative delay calculation method that substantially reduces storage requirements. We briefly describe how the separable beamformer can be implemented in the context of Sonic Millip3De, our recently proposed hardware accelerator for the digital front-end of a 3-D ultrasound system.