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High-entropy materials containing a mixture of five or more elemental species represent a paradigm shift in materials science where a variety of alloys, oxides, carbides, nitrides, and borides can be synthesized with superior physical and mechanical properties than are accessible from the constituent materials. We report on the high-pressure high-temperature synthesis of high-entropy transition metal borides and thermal equation of state measurements to 9.5 GPa and 2273 K at beamline 16-BM-B, HPCAT, Advanced Photon Source, Argonne National Laboratory. High-entropy material (Hf0.2 Ti0.2 Nb0.2 Ta0.2 Mo0.2)B2 was synthesized starting from ball-milled oxide precursors of constituent metals mixed with graphite and boron-carbide. The synthesized material crystalized in hexagonal AlB2 structure which remained stable to the highest pressure and temperature achieved in this study. Microstructural and nanoindentation hardness data was obtained from measurements on the recovered sample and thermal expansion data was obtained from the equation of state studies at high temperatures. High shear strength and phase stability under high-pressure high-temperature conditions make these materials ideal for extreme environments.