Volumetric, multimodal imaging with precise spatial and temporal coregistration can provide valuable and complementary information for diagnosis and monitoring. Considerable research has sought to combine 3-D photoacoustic (PA) and ultrasound (US) imaging in clinically translatable configurations; however, technical compromises currently result in poor image quality either for PA or ultrasonic modes. This work aims to provide translatable, high-quality, simultaneously coregistered dual-mode PA/US 3-D tomography. Volumetric imaging based on a synthetic aperture approach was implemented by interlacing PA and US acquisitions during a rotate-translate scan with a 5-MHz linear array (12 angles and 30-mm translation to image a 21-mm diameter, 19 mm long cylindrical volume within 21 s). For coregistration, an original calibration method using a specifically designed thread phantom was developed to estimate six geometrical parameters and one temporal offset through global optimization of the reconstructed sharpness and superposition of calibration phantom structures. Phantom design and cost function metrics were selected based on analysis of a numerical phantom and resulted in a high estimation accuracy for the seven parameters. Experimental estimations validated the calibration repeatability. The estimated parameters were used for the bimodal reconstruction of additional phantoms with either identical or distinct spatial distributions of US and PA contrasts. The superposition distance of the two modes was within < 10% of the acoustic wavelength, and a wavelength-order uniform spatial resolution was obtained. This dual-mode PA/US tomography should contribute to more sensitive and robust detection and follow-up of biological changes or the monitoring of slower-kinetic phenomena in living systems such as the accumulation of nanoagents.