A combined photoacoustic-ultrasound (PAUS) tomography system is introduced using ring-array and novel aperture encoding schemes. The ultrasound subsystem is able to achieve diffraction limited half-wavelength isotropic in-plane spatial resolution unlike previous systems. S-sequence aperture encoding improves signal-to-noise ratio (SNR) for the ultrasound tomography (UST) subsystem. We measured an average resolution of [Formula: see text] for S-sequence UST and a resolution of [Formula: see text] for photoacoustic tomography. We were able to measure SNR improvement using S-sequence spatial encoding using tissue-mimicking phantoms, and we displayed a composite PAUS phantom image.