Abstract: Fe–Cr–Ni austenitic alloys are extensively utilized in the hot-end components of nuclear light water reactors, turbine disks, and gas compressors. However, their low strength at elevated temperatures limits their engineering applications. In this study, a novel precipitation-strengthened alloy system is developed by incorporating Al and Si elements into a FeCrNi equiatomic alloy. The results indicate that the FeCrNiAl_xSi_x (at%, x = 0.1, 0.2) alloys possess heterogeneous precipitation structures that feature a micron-scale σ phase at the grain boundaries and a nanoscale ordered body-centered cube (B2) phase within the grains. An exceptional strength–ductility synergy across a wide temperature range is achieved in FeCrNiAl_0.1Si_0.1 alloys due to grain refinement and precipitation strengthening. Notably, a yield strength of 693.83 MPa, an ultimate tensile strength of 817.55 MPa, and a uniform elongation of 18.27% are attained at 873 K. The dislocation shearing mechanism for B2 phases and the Orowan bypass mechanism for σ phase, coupled with a high density of nano-twins and stacking faults in the matrix, contribute to the excellent mechanical properties at cryogenic and ambient temperatures. Moreover, the emergence of serrated σ phase and micro-twins in the matrix plays a crucial role in the strengthening and toughening mechanisms at intermediate temperatures. This study offers a novel perspective and strategy for the development of precipitation-hardened Fe–Cr–Ni austenitic alloys with exceptional strength–ductility synergy over a broad temperature range.