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Compared to other body-centered cubic (bcc) transition metals, Nb has been the subject of fewer compression studies and there are still aspects of its phase diagram which are unclear. Here, we report a combined theoretical and experimental study of Nb under high pressure and temperature. We present the results of static laser-heated diamond anvil cell experiments up to 120 GPa using synchrotron-based fast x-ray diffraction combined with ab initio quantum molecular dynamics simulations. The melting curve of Nb is determined and evidence for a solid-solid phase transformation in Nb with increasing temperature is found. The high-temperature phase of Nb is orthorhombic Pnma. The bcc-Pnma transition is clearly seen in the experimental data on the Nb principal Hugoniot. The bcc-Pnma coexistence observed in our experiments is explained. Agreement between the measured and calculated melting curves is very good except at 40-60 GPa where three experimental points lie below the theoretical melting curve by 250 K (or 7%); a possible explanation is given. The study of materials under extreme conditions can reveal interesting physics in diverse areas such as condensed matter and geophysics. Here, the authors investigate experimentally and theoretically the high pressure-high temperature phase diagram of niobium revealing a previously unobserved phase transition from body-centered cubic to orthorhombic phase. ; Funding Agencies|Spanish Ministerio de Ciencia, Innovacion y Universidades [MAT2016-75586-C4-1-P, PGC2018-097520-A-100, RED2018-102612-T]; Generalitat ValencianaGeneralitat Valenciana [Prometeo/2018/123 EFIMAT]; Spanish governmentSpanish Government [RyC-2014-15643]; US Department of EnergyUnited States Department of Energy (DOE) [DE-AC52-07NA27344]