Materials scientists studying the coarsening of crystalline solids have long speculated that individual crystallites might undergo rigid-body rotations along with the shrinkage or growth caused by boundary migration. The driving force for such grain rotations would be a reduction in interfacial energy: if two grains can find a way to rotate toward each other’s orientation, then the misorientation at their common boundary will decrease in magnitude or even vanish, which in turn will lower the free energy of the system. Investigations of spherical grains sintered onto flat, single-crystalline substrates have lent credibility to this hypothesis; however, it remains to be determined whether grain rotations also take place under technically relevant conditions, such as the sintering of closely packed powders or the Ostwald ripening of a two-phase material. Employing the still-novel technique of three-dimensional x-ray diffraction (3DXRD) microscopy, we have detected the occurrence of grain rotations in a bulk, semisolid Al-Cu alloy undergoing Ostwald ripening. Not only did we find the average grain rotation to increase with relative volume of the liquid phase (as one might naively expect), but we also established that the orientation dependence of the interfacial energy biases the direction of rotation. These results suggest that grain rotations may influence the densification and coarsening of a range of systems, with corresponding implications for the accurate modeling of sintering and of materials processing in the semisolid state.