In this study, a novel cobalt-free Fe40Mn20Cr20Ni20 (at.%) medium-entropy alloy (MEA) with low cost is purposively designed. The partially recrystallized microstructure was introduced into the single-phase FeMnCrNi MEA via thermo-mechanical processing. Surprisingly, the present MEA displays ultrahigh yield and ultimate tensile strengths of 1.2 and 1.34 GPa, respectively, and large ductility with a uniform elongation of 22% at 77 K. This is mainly due to the accumulation and entanglement of a large number of dislocations and the activation of deformation twins at 77 K. •A novel cobalt-free Fe40Mn20Cr20Ni20 medium-entropy alloy with low cost is purposively designed.•The present medium-entropy alloy displays ultrahigh yielding and ultimate tensile strengths of 1.2 and 1.34 GPa, respectively, and good ductility with a uniform elongation of 22% at 77 K.•Compared with many other advanced alloys, the present MEA has outstanding advantages in the mechanical performances at 77 K and the cost of raw materials for the preparation.