By using a diamond-anvil cell (DAC) with laser heating technology, a cubic perovskite polymorph of SrSiO3 has been synthesized at ~38 GPa and 1500–2000 K for the first time. The P-V data of this new phase give ambient temperate elastic constants of V0 = 49.18(5) Å3, K0 = 211(3) GPa, respectively, when they are fitted against the Birch-Murnaghan equation of state with a fixed K0′ at 4. On decompression, the SrSiO3 cubic perovskite phase becomes unstable at ~6.2 GPa and disappears completely at ~4.7 GPa. The transformed product can be considered as an amorphous phase with a minor amount of small sized crystals in the amorphous matrix. First principle calculations predicted structural properties of both the cubic and the six-layer-repeated hexagonal perovskite polymorphs of SrSiO3 in good agreement with experimental results. The experimental and theoretical results indicate that the larger Sr2+ cation can substitute the Ca2+ cation and enter into the lattice of the cubic perovskite phase of CaSiO3 at lower mantle conditions with only a small lattice strain. These results indicate that Sr can be hosted in cubic perovskite CaSiO3 found as inclusions in diamonds originating from the lower mantle.