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MgGeO₃ post‐perovskite (pPv) is a lower pressure analogue of MgSiO₃‐pPv, which is believed to be the main constituent of the Earth's D″ layer. Understanding the physical properties of this phase is critical to explain seismological observations as seismic anisotropy is likely linked to lattice preferred orientation in post‐perovskite, which is governed by the motion of defects such as dislocations. Here, we apply in‐situ three‐dimensional X‐ray diffraction to a polycrystalline sample of MgGeO₃‐pPv at 90 GPa. We demonstrate how the method can be used to follow individual grains within the material, including their individual orientations, positions, and strain tensors. We then use X‐ray line profile analysis to characterize dislocations in the grains. The most probable slip systems are <110> gliding on or (001) and [110](001), with a smaller contribution of [100](001) and [001] dislocations. These slip systems are …