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CsYbSe2 has an ideal triangular-lattice geometry with pronounced two-dimensionality, pseudospin-1/2 nature, and the absence of structural disorder. These excellent characteristics favor a quantum spin-liquid realization in this material. In this work, we applied quasihydrostatic compression methods to explore the structural behaviors. Our study reveals that CsYbSe2 undergoes a structural transition around 24 GPa, accompanied by a large volume collapse of ΔV /V0∼13%. The ambient hexagonal structure with the space group P63/mmcis lowered to the tetragonal structure (P4/mmm) under high pressure. Meanwhile, the color of CsYbSe2 changes gradually from red to black before the transition. Dramatic pressure-induced changes are clarified by the electronic structure calculations from the first principles, which indicate that the initial insulating ground state turns metallic in a squeezed lattice. These findings highlight Yb-based dichalcogenide delafossites as an intriguing material to probe novel quantum effects under high pressure.