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Lead halide perovskites have a rich landscape of structural and optical properties, which can be explored and possibly controlled by applying high pressure. Despite several reports on high-pressure studies of CsPbBr3nanocrystals (NCs), there have so far been no studies under pressure that incorporate planar defects. CsPbBr3NCs with Ruddlesden–Popper (RP) faults, formed via post-synthetic fusion growth, are significantly larger in size than as-synthesized NCs and display exceptional emission stability. Here, we compare synchrotron-based high-pressure X-ray diffraction and photoluminescence (PL) properties of CsPbBr3(without RP) and RP-CsPbBr3(with RP) and resolve their crystal structure under pressure for the first time. CsPbBr3undergoes a phase transition from the orthorhombic Pnmaphase at ambient pressure to the cubic Pm3̅mphase at 1.7 GPa, and RP-CsPbBr3transforms from Pnmato the monoclinic P21/mphase at 0.74 GPa in addition to several isostructural transitions. Density-functional calculations predict a narrowing of the band gap with pressure, concomitant with the PL energies. The RP-CsPbBr3NCs exhibit enhanced PL intensity at 1 GPa and show band gap opening at high pressures. This study opens new strategies for not only tuning just the structural properties but also tuning planar defects in alkali halide lead crystals for improved optical properties.