Iron-bearing silicate perovskite is believed to be the most abundant mineral of the Earth's lower mantle. Recent studies have shown that Fe²⁺ exists predominantly in the intermediate-spin state with a total spin number of 1 in silicate perovskite in the lower part of the lower mantle. Here we have measured the spin states of iron and the pressure–volume relation in silicate perovskite [(Mg_0.6,Fe_0.4)SiO₃] at pressure conditions relevant to the lowermost mantle using in situ X-ray emission and X-ray diffraction in a diamond cell. Our results showed that the intermediate-spin Fe²⁺ is stable in the silicate perovskite up to ∼ 125 GPa but starts to transition to the low-spin state at approximately 135 GPa. Concurrent X-ray diffraction measurements showed a decrease of approximately 1% in the unit cell volume in the silicate perovskite [(Mg_0.6,Fe_0.4)SiO₃], which is attributed to the intermediate-spin to the low-spin transition. The transition pressure coincides with the pressure conditions of the lowermost mantle, raising the possibility of the existence of the silicate perovskite phase with the low-spin Fe²⁺ across the transition from the post-perovskite to the perovskite phases in the bottom of the D″ layer.