We report on our first application of synchrotron-based X-ray microtomography (SXRT) to image samples of young sea-water ice grown in a tank. To conserve and image the pore network structure of sea ice in situ we applied: (1) rapid centrifugation of brine at in situ ice temperatures; (2) storage and transport of centrifuged samples on dry ice; and (3) tomographic imaging on a cooling stage keeping the sample below the eutectic temperature of NaCl. The technique yields nondestructive three-dimensional images of the sea-ice microstructure with micrometer resolution, improving on previous mostly two-dimensional studies by one to two orders of magnitude (voxel sizes from 0.7 to 6.0 microns). Besides a clear discrimination of connected and disconnected pore space, SXRT provides unprecedented details of the sea-ice pore morphology. Here we describe the morphology of some known characteristic sea-ice length and pore scales. We focus on the sea-ice plate spacing (0.3–0.6 mm), on wide brine channels with average diameters of 1–2 mm and on thin O (0.1 mm) sheet-like pore networks located in planes between the vertically oriented ice plates. The transition of lamellar brine layers – the well-known near-interface morphology of sea ice – into the latter networks is of particular importance for the physical properties of sea ice. We conclude that SXRT has a large potential to improve our understanding of the microstructural evolution of sea ice, with implications for proper modeling of salt entrapment and release.