In this study, we investigate the precipitation kinetics of Cu in 15–5 PH maraging stainless steel during high-temperature thermal treatments in the fully austenitic state. This provides direct evidence that Cu precipitation can occur in the austenite phase of martensitic or ferritic steels. The kinetics of Cu precipitation in austenite are examined at 700 and 800 °C using in situ synchrotron small-angle and wide-angle X-ray scattering, complemented by atom probe tomography investigations to analyze the precipitates, particularly their chemistry, following heat treatment. The resulting experimental data, which include the evolution of size, volume fraction, number density and chemical composition, are used to inform precipitation kinetics modelling using the Langer-Schwartz-Kampmann-Wagner (LSKW) approach coupled with CALPHAD thermodynamic and kinetic databases. The simulations accurately capture the experimental data by adjusting the interfacial energy in an inverse modelling approach. The insight that Cu precipitation occurs in austenite and subsequently in martensite paves the way for design of hierarchical structures with a bi-modal particle size distribution of Cu precipitates with varying crystal structures and compositions. Additionally, the validated LSKW modelling approach establishes a foundation for designing Cu-alloyed high-performance steels, taking into account various manufacturing routes.