Abstract Periodic mesoporous hexagonal boron nitride was investigated as a precursor for nanoscale forms of cubic boron nitride. Cubic boron nitride (c‐BN) nanocrystals were obtained at a pressure of 14 GPa and a temperature of 1300 °C. The synthesized nanocrystals have diameters of ∼ 50 nm and are solution‐processible. Mesoporous c‐BN formed at a pressure of 10 GPa and 1000 °C. The mesoporous c‐BN has a pore size distribution centered around 3 nm and a surface area of 122 m 2 /g as determined by electron tomography. At pressures of 8 and 6 GPa near monodisperse h‐BN nanodiscs with diameters of 0.5‐1 μm and 0.2‐0.5 μm were formed, respectively. The discs formed colloidal solutions in acetone. The hydrodynamic radius of the discs matched the radii determined by dynamic light scattering indicating absence of nanodisc aggregation in solution.