The phase diagram of copper is investigated at pressures ranging from 73 to 177 GPa and temperatures up to 6000 K using free electron laser X-rays in a serial pump-probe mode in diamond anvil cells. X-ray diffraction measurements confirm and delimit the stability domain of the bcc-Cu phase at high‑pressure and high‑temperature; a new phase {with a plausible 9R structure} is also observed. The Gibbs free energies of relevant crystalline phases are computed over ranges of 0–500 GPa and 300–8000 K, respectively, using machine-learning accelerated thermodynamic integration within an {} framework. Integrating the present theoretical and experimental results, we construct a comprehensive thermodynamic phase diagram of Cu {with stability domains for bcc and fcc phases} and infer that 9R is a metastable phase produced as a consequence of rapid quenching in the X-ray heated copper sample.