The use of nanoscale x-ray probes overcomes several key limitations in the study of materials up to multimegabar (> 200) pressures, namely, the spatial resolution of measurements of multiple samples, stress gradients, and crystal domains in micron to submicron size samples in diamond-anvil cells. Mixtures of Fe, Pt, and W were studied up to 282 GPa with 250-600 nm size synchrotron x-ray absorption and diffraction probes. The probes readily resolve signals from individual materials, between sample and gasket, and peak pressures, in contrast to the 5-microm-sized x-ray beams that are now becoming routine. The use of nanoscale x-ray beams also enables single-crystal x-ray diffraction studies in nominally polycrystalline samples at ultrahigh pressures, as demonstrated in measurements of (Mg,Fe)SiO(3) postperovskite. These capabilities have potential for driving a push toward higher maximum pressures and further miniaturization of high-pressure devices, in the process advancing studies at extreme conditions.