The crystal structure of the ordered double perovskite Ba2PrRu0.8Ir0.2O6 was investigated as a function of pressure at room temperature. High-resolution synchrotron powder diffraction measurements have revealed the occurrence of a first order phase transition at a critical pressure of about 0.5 GPa. Structural refinements indicate that Ba2PrRu0.8Ir0.2O6 transforms from an ambient pressure monoclinic (P 21/n) to a high-pressure tetragonal (P4/mnc) structure. The transition is consistent with a valence change of the Pr ions.