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The stishovite to post‐stishovite phase transition may modify the scattering of seismic waves by stishovite‐bearing rocks in the Earth's lower mantle. A series of continuous compression experiments on sintered polycrystalline stishovite was performed to study the effect of stress on the phase transition. The experimental results show that the phase transition shifts to lower pressures as the magnitude of deviatoric stress increases. Our results further show that the bulk modulus of sintered polycrystalline stishovite differs from that derived from single crystal measurements and decreases at the phase transition. In cold regions, such as subducted slabs, stresses may accumulate and shift the phase transition to a shallower depth. In hot regions with less stress, such as rising plumes, the phase transition is shifted to a greater depth. In addition, the phase transition may have varying seismic signatures depending on the behavior of the grain boundaries in mantle rocks and the micro‐stresses present in neighboring grains. Plain Language Summary: Stishovite and post‐stishovite are high‐pressure polymorphs of SiO2 that are stable in the Earth's lower mantle. The stishovite to post‐stishovite phase transition may affect the way seismic waves travel through mantle rocks. To detect SiO2 phases in the deep mantle, it is essential to investigate the depth of the phase transition and its effects on seismic wave propagation. Experiments were conducted to simulate the high‐pressure environment of the lower mantle and to study the phase transition of sintered polycrystalline stishovite aggregates, in which crystals are firmly connected to each other. Our study finds that the phase transition is sensitive to deviatoric stress, which is a measure of the directional stress difference. An increase in deviatoric stresses shifts the phase transition to shallower depths in the mantle. We further find that at pressures below the phase transition, sintered polycrystalline stishovite aggregates exhibit lower compressibility compared to stishovite powder and single crystals, but the compressibility increases abruptly at the phase transition, probably caused by grain‐grain interactions in the sintered samples. Therefore, the behavior of seismic waves will be different when passing through stishovite‐bearing rocks with different degree of grain‐grain interactions, particularly under conditions close to the phase transition boundary. Key Points: Continuous compression experiments were performed to study the post‐stishovite phase transition under different stress conditionsDeviatoric stresses shift the phase transition to a shallower depth in the lower mantleThe bulk modulus of sintered polycrystalline stishovite differs from that of stishovite powder and drops at the phase transition [ABSTRACT FROM AUTHOR]