AbstractThe occurrence of buetschliite, K₂Ca(CO₃)₂, as inclusions in mantle minerals, is considered as one of the keys to understanding phase relationships of dense carbonates and outlines the potential role of potassium carbonates in the Earth's deep carbon cycle. Within this scope, the high‐pressure behavior of synthetic buetschliite is characterized by in situ Raman spectroscopy up to 19 GPa and 300°C. Up to 6 GPa, the compression is regular, then the splitting of some of the lattice and internal modes defines the transition to a low‐symmetry phase, in analogy to that observed previously in K₂Mg(CO₃)₂. The temperature rise to 300°C shifts the transition pressure from ~6 to ~8 GPa, but on the whole, it does not change the high‐pressure behavior of K₂Ca(CO₃)₂. The observed pressure‐induced spectral changes are fully reversible at room and elevated temperature. The findings show the expansion of buetschliite baric stability with temperature, which confirms its importance as a constituent of carbonate inclusions in deep minerals.