<p>The layered materials typically exhibit greater compressive behavior along the out-of-plane direction, and thus pressure typically leads to the collapse of the c-axis lattice constant and promotes the system becoming a more three-dimensional nature at high pressure. Here, based on high-pressure electrical transport and XRD measurements, we uncover a unique pressure-induced insulator-to-metal transition occurring in the altermagnet La₂O₃Mn₂Se₂, which is accompanied by unusual in-plane lattice collapse instead of conventional out-of-plane collapse. Such a transition can be attributed to the spin-state transition of Mn²⁺ from a high-spin (HS, S = 5/2) state to an intermediate-spin (IS, S = 3/2) state, forming a pressure-driven cooperative spin crossover. Moreover, we find that the pressure-driven Mott phase transition in La₂O₃Mn₂Se₂ exhibits orbital-selective characteristics. Our findings reveal, to the best of our knowledge, the first observation of the in-plane lattice collapse of layered materials under pressure, without a symmetry change in the structure. This sheds light on the understanding of high-pressure physics and phase transitions of layered materials and provides a unique model for pressure-driven spin-crossover multifunctional materials.</p>