Polycrystalline cubic boron nitride (PcBN) has been synthesized at 14 GPa and high temperatures of 1300–2000 °C in a two-stage multi-anvil cell. Sintered PcBN synthesized at 1700–1800 °C and 14 GPa with a grain size of ∼200 nm is optically transparent with a transmittance of ∼70% at wavelengths of 400–1500 nm and has the Vickers hardness of ∼63–69 GPa. Analyses with scanning and transmission electron microscopy reveal that PcBN can be strengthened by introducing nanometer-scale grains and microscopic defects at high pressure and temperature. The optical transparency of the bulk PcBN synthesized at high pressure and temperature can be explained by the very thin intergranular films between grains. The present sintered PcBN is the second-hardest transparent material after diamond and can be used for windows in extreme conditions.