The transport of calcium carbonate (CaCO3) into the Earth’s interior through subduction is one of the key processes in the global cycling of carbon. To develop a better understanding of the CaCO3 structural stability during subduction processes, the phase transitions among CaCO3-I (calcite), CaCO3-II, -III/IIIb, and aragonite under pressure-temperature (P-T) conditions up to 2.5 GPa and 600 °C, in hydrous and anhydrous environments, were investigated using a hydrothermal diamond-anvil cell. One displacive and two reconstructive processes during the phase transitions among CaCO3 polymorphs were confirmed from the results obtained from in situ observations and Raman spectroscopic measurements. Meanwhile, the effect of Ca-substitutional metal cations (e.g., Mg2+) in CaCO3 and the presence of an aqueous fluid on the phase transition processes have been determined. Specifically, the CaCO3-I ↔ -II phase transition is a displacive process, occurring instantly at pressures varying from 1.6 GPa at room temperature to 1.5 GPa at 500 °C with the phase equilibrium boundary having a minimum P-T point at ~1.4 GPa at 300 °C, and is completely reversible upon cooling and decompression. The CaCO3-II → -III phase transition is a reconstructive process, observed at P-T conditions from 2.0 GPa at room temperature to 2.5 GPa at 150 °C, and is accomplished by solid recrystallization starting from CaCO3-II, transitioning through an intermediate CaCO3-IIIb, and ending at the CaCO3-III structure. The phase transition between CaCO3-I or -II and aragonite, which is also a reconstructive process, was found to occur by progressive solid recrystallization under high P-T hydrous and anhydrous conditions, or alternatively, via dissolution-precipitation under low-P-T hydrous conditions, depending on the presence of aqueous fluids and the heating rate of the system. The substitution for Ca2+ by other metal cations (e.g., Mg2+, Mn2+, Fe2+) in CaCO3 results in a significant increase in the pressures for the displacive and solid recrystallization reconstructive phase transitions, but has no detectable influence on the CaCO3-I/II ↔ aragonite transformation via a dissolution-precipitation process under hydrous conditions. Our results show that the presence of Ca-substitutional metal cations in CaCO3 is a key factor controlling the phase stability of CaCO3 under high P-T conditions, and suggest that aragonite should be the predominant phase in the upper mantle in subduction zones where the heating rate is very low and slab dehydration is prevalent.