Hydrous Fe3+‐bearing bridgmanite (Bdg) is potentially a critical water host in the lowermostmantle. The spin transition behaviors of such materials are pivotal for understanding geophysicalheterogeneity in the deep Earth but are poorly understood. Here, we investigated the spin transition andrelated geophysical properties of Fe3+ with associated H defects [Fe3+‐H] at high P‐T conditions using first‐principles simulations. Our calculations predict that the presence of hydrogen reduces the onset pressure ofthe spin transition of Fe3+ in Bdg, leading to higher fractions of low spin Fe3+. Along standard geotherms,spin transition is predicted to remain incomplete even at the core‐mantle boundary (CMB), and lateraltemperature variations would significantly affect the proportions of high and low spin Fe and relatedproperties like elasticity. The thermoelastic property of hydrous Fe3+‐bearing bridgmanite exhibit strongersoftening anomalies at the lower mantle conditions compared to dry system, which potentially enhancing theseismic detectability of the hydrous Bdg in the deep earth. Density profile of hydrous Fe3+‐bearingbridgmanite indicates that the [Fe3+‐H] defect modestly increases the system's density, but much less than thatcaused by incorporating an equivalent amount of iron alone. This is crucial for understanding regions likeLarge Low Shear Velocity Provinces (LLSVPs), which exhibits large velocity drops but only minor densitychanges. The co‐adsorption of Fe and H allows for the introduction of Fe to induce velocity drops without theconcomitant sharp increase in density, as pure iron would, thus enabling Fe‐H enrichment as a potentialsource of LLSVPs.