The high-pressure behaviour of manganese carbonate was investigated by in situ synchrotron X-ray powder diffraction up to 54 GPa with a laser-heated diamond anvil cell. A phase transition from rhodochrosite to a new structure form was observed at 50 GPa after laser heating. The diffraction pattern of the new high-pressure form was reasonably indexed with an orthorhombic unit-cell with a = 5.361 Å, b = 8.591 Å and c = 9.743 Å. The pressure-induced phase transition implies a unit-cell volume reduction of ~5%. This result does not support the direct formation of diamond by dissociation of solid state MnCO3 reported in a previous study. Fitting the compression data of rhodochrosite to a second-order Birch-Murnaghan equation of state (K0′ = 4) gives K0 = 126(±10) GPa. The c axis of the unit-cell parameter was more compressive than the a axis.