A Cu precipitation-mediated austenitic transformation during ageing treatment of a 15 & ndash;5 PH stainless steel is revealed through atom probe tomography, in situ synchrotron X-ray diffraction and computational thermodynamics and kinetics. The austenitic transformation is proposed to occur through the pathway: Cu precipitation at the martensite/retained austenite interfaces or at martensite lath boundaries -> partitioning of austenite stabilizing elements towards interfaces of the Cu precipitates -> reverted austenite formation.