CsCoO2, featuring a two‐dimensional layered architecture of edge‐ and vertex‐linked CoO4 tetrahedra, is subjected to a temperature‐driven reversible second‐order phase transformation (α → β) at 100 K, which corresponds to a structural relaxation with concurrent tilting and breathing modes of edge‐sharing CoO4 tetrahedra. In the present investigation, it was found that pressure induces a phase transition, which encompasses a dramatic change in the connectivity of the tetrahedra. At 923 K and 2 GPa, β‐CsCoO2 undergoes a first‐order phase transition to a new quenchable high‐pressure polymorph, γ‐CsCoO2. It is built up of a three‐dimensional cristobalite‐type network of vertex‐sharing CoO4 tetrahedra. According to a Rietveld refinement of high‐resolution powder diffraction data, the new high‐pressure polymorph γ‐CsCoO2 crystallizes in the tetragonal space group I41/amd:2 (Z = 4) with the lattice constants a = 5.8711 (1) and c = 8.3214 (2) Å, corresponding to a shrinkage in volume by 5.7% compared with the ambient‐temperature and atmospheric pressure β‐CsCoO2 polymorph. The pressure‐induced transition (β → γ) is reversible; γ‐CsCoO2 stays metastable under ambient conditions, but transforms back to the β‐CsCoO2 structure upon heating to 573 K. The transformation pathway revealed is remarkable in that it is topotactic, as is demonstrated through a clean displacive transformation track between the two phases that employs the symmetry of their common subgroup Pb21a (alternative setting of space group No. 29 that matches the conventional β‐phase cell). [ABSTRACT FROM AUTHOR]