The high-pressure behavior of manganese-rich carbonate, rhodochrosite, has been characterized up to 62 GPa by synchrotron-based midinfrared spectroscopy and X-ray diffraction. Modifications in both the infrared spectra and the X-ray diffraction patterns were observed above ~35 GPa, indicating the presence of a high-pressure phase transition at these pressures. We found that rhodochrosite adopts a structure close to CaCO3-VI with a triclinic unit cell (a = 2.87 Å, b = 4.83 Å, c = 5.49 Å, α = 99.86°, β = 94.95°, and γ = 90.95° at 62 GPa). Using first-principles calculations based on density functional theory, we confirmed these observations and assigned modes in the new infrared signature of the high-pressure phase. These results suggest that high-pressure metastable phase of calcite may play an important role in carbon storage and transport in the deep Earth.