High-density stacking faults (SFs) were introduced into a novel Ni−Co-based superalloy through warm rolling at 300−500 °C, and the effects of SFs on its tensile properties at intermediate temperatures (650 and 750 °C) were investigated. The results indicated that all warm rolled specimens have high-density SFs and Lomer−Cottrell locks compared with the initial specimens. Meanwhile, the simultaneous improvement of intermediate-temperature strength and ductility of alloys can be achieved by high-density SFs. In particular, the specimen rolled at 300 °C exhibited a superior combination of high strength (yield and ultimate tensile strengths of (1311±18) and (1462±25) MPa respectively at 650 °C, and (1180±17) and (1293±15) MPa respectively at 750 °C) and high fracture elongation ((26.7±2.5)% at 650 °C and (10.7±1.3)% at 750 °C). The high strengths and facture elongations of all warm-rolled specimens were primarily attributed to the interaction of pre-existing γ′ phases, high-density SFs and Lomer−Cottrell locks with dislocations, as well as to the formation of high-density deformation nano-twins during tensile loading.