<p>Alloy 625 is a mature Ni-based superalloy widely used in the aerospace and chemical industries, but its qualification for nuclear applications requires a better understanding of its long-term stability and viscoplastic behavior (<i>i.e.,</i> time-dependent deformation). This study investigates the effect of thermal aging at 700&#xa0;°C (100, 1000, 2664, and 4107&#xa0;hours) on the microstructural evolution and the resulting creep and stress relaxation behavior of Alloy 625. Microstructural analyses revealed an initial state without detectable precipitates; with increasing aging time at 700&#xa0;°C, progressive precipitation of strengthening <i>γ</i>″ (Ni₃Nb, D0₂₂) occurred. With further exposure (&gt;&#xa0;1000&#xa0;hours), <i>γ</i>″ progressively transforms into <i>δ</i> phase (Ni₃Nb, D0a), strongly influenced by Nb/Mo chemical segregation, while Nb/Mo-rich carbides slightly coarsened at grain boundaries. Mechanical testing (tensile, creep and stress relaxation) demonstrated that short-term aging (100&#xa0;hours) improves viscoplastic resistance through <i>γ</i>″ hardening, whereas prolonged exposure leads to a two-order-of-magnitude increase in strain rate at 300&#xa0;MPa, reflecting reduced viscoplastic resistance due to <i>δ</i> formation at the expanse of <i>γ</i>″ particles. Despite this degradation, Alloy 625 retained high creep ductility (&gt;&#xa0;40&#xa0;pct) under all conditions, and segregation bands did not significantly affect monotonic creep performance. Stress relaxation tests provided consistent trends with creep once internal stresses were relieved, offering an efficient means to identify viscoplastic parameters. Overall, these results clarify the link between microstructural transformations and viscoplastic behavior in Alloy 625, support its assessment for nuclear service at 700&#xa0;°C, and provide a basis for the development of microstructure-sensitive constitutive models.</p>