The high-pressure phase relation of MnSiO3 was examined up to 85 GPa and 2600 K using a laser-heated diamond-anvil cell combined with synchrotron radiation. MnSiO3 garnet decomposes into a mixture of MnO with a rock-salt structure (B1) + SiO2 stishovite at pressures higher than ~20 GPa and temperatures higher than ~1200 K. However, MnO (B1) + SiO2 stishovite further transforms to a perovskite structure with increasing pressure. The phase boundary between these structures is positive in the pressure-temperature diagram. The triple point of garnet, MnO + SiO2 and perovskite in the pressure-temperature diagram is ~20 GPa and 1200 K. MnSiO3 perovskite is orthorhombic, and consistent with space group Pbnm, both at high pressure and high temperature and at high pressure and room temperature, but becomes amorphous during decompression. The refined cell parameters of MnSiO3 perovskite at 85 GPa and 2600 K are a = 4.616(2) Å, b = 4.653(2) Å, c = 6.574(3) Å, and V = 141.2(2) Å3. The a/b ratio increases (approaches 1) with pressure and temperature, while the √2a/c ratio remains nearly constant (<1). This indicates that the orthorhombic distortion decreases and the structure tends toward a tetragonal perovskite with increasing pressure and temperature.