The origin of the extraordinary strengthening of the highly alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700 °C was investigated by the use of advanced scanning transmission electron microscopy (STEM). Along with substantial change of the dislocation structure, nucleation of two distinct populations of nanoparticles was revealed. Fully coherent Cu-rich nanoparticles were observed to be homogeneously dispersed with high number density along with nanometer-sized incoherent NbC carbides precipitating on dislocations during cyclic loading. Probe-corrected high-angle annular dark-field STEM imaging was used to characterize the atomic structure of nanoparticles. Compositional analysis was conducted using both electron energy loss spectroscopy and high spatial resolution energy dispersive X-ray spectroscopy. High-temperature exposure-induced precipitation of spatially dense coherent Cu-rich nanoparticles and strain-induced nucleation of incoherent NbC nanoparticles leads to retardation of dislocation movement. The pinning effects and associated obstacles to the dislocation motion prevent recovery and formation of the localized low-energy cellular structures. As a consequence, the alloy exhibits remarkable cyclic hardening at elevated temperatures.