A newly developed P-doped CrCoNi medium-entropy alloy (MEA) provides both higher yield strength and larger uniform elongation than the conventional CrCoNi MEA, even superior tensile ductility to the otherelement-doped CrCoNi MEAs at similar yield strength levels. P segregation at grain boundaries (GBs) and dissolution inside grain interiors, together with the related lower stacking fault energy (SFE) are found in the P-doped CrCoNi MEA. Higher hetero-deformation-induced (HDI) hardening rate is observed in the Pdoped CrCoNi MEA due to the grain -to -grain plastic deformation and the dynamic structural refinement by high-density stacking fault-walls (SFWs). The enhanced yield strength in the P-doped CoCrNi MEA can be attributed to the strong substitutional solid -solution strengthening by severer lattice distortion and the GB strengthening by phosphorus segregation at GBs. During the tensile deformation, the multiple SFW frames inundated with massive multi-orientational tiny planar stacking faults (SFs) between them, rather than deformation twins, are observed to induce dynamic structural refinement for forming parallelepiped domains in the P-doped CoCrNi MEA, due to the lower SFE and even lower atomically-local SFE. These nano-sized domains with domain boundary spacing at tens of nanometers can block dislocation movement for strengthening on one hand, and can accumulate defects in the interiors of domains for exceptionally high hardening rate on the other hand. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.