A medium-entropy alloy, CrCoNi, exhibits exceptional damage tolerance at cryogenic temperatures. This alloy, composed of three elements in equiatomic proportions, forms a single-phase face-centred cubic (fcc) solid solution. At room temperature, it displays high tensile strength (almost 1 GPa), high failure strain (around 70%), and fracture toughness (KJIC values above 200 MPa m^1/2). At cryogenic temperatures (77 K), its strength increases to over 1.3 GPa, failure strain reaches up to 90%, and KJIC values reach 275 MPa m^1/2. These properties are attributed to continuous steady strain hardening, which suppresses plastic instability through dislocation activity and deformation-induced nano-twinning. CrCoNi outperforms other high-entropy and multi-phase alloys in terms of strength, ductility, and fracture toughness. Its unique combination of properties is due to its single-phase microstructure and the presence of nano-twinning, which enhances damage tolerance. The alloy's performance is further improved at lower temperatures, making it one of the toughest metallic materials reported to date. The study highlights the importance of alloy composition and microstructure in achieving exceptional mechanical properties, and demonstrates that the nature of the alloying elements is more important than their sheer number. The CrCoNi alloy is compared to other materials, including high-entropy alloys and twinning-induced plasticity steels, and is found to have superior mechanical properties. The research provides insights into the mechanisms underlying the exceptional damage tolerance of the CrCoNi alloy at cryogenic temperatures.A medium-entropy alloy, CrCoNi, exhibits exceptional damage tolerance at cryogenic temperatures. This alloy, composed of three elements in equiatomic proportions, forms a single-phase face-centred cubic (fcc) solid solution. At room temperature, it displays high tensile strength (almost 1 GPa), high failure strain (around 70%), and fracture toughness (KJIC values above 200 MPa m^1/2). At cryogenic temperatures (77 K), its strength increases to over 1.3 GPa, failure strain reaches up to 90%, and KJIC values reach 275 MPa m^1/2. These properties are attributed to continuous steady strain hardening, which suppresses plastic instability through dislocation activity and deformation-induced nano-twinning. CrCoNi outperforms other high-entropy and multi-phase alloys in terms of strength, ductility, and fracture toughness. Its unique combination of properties is due to its single-phase microstructure and the presence of nano-twinning, which enhances damage tolerance. The alloy's performance is further improved at lower temperatures, making it one of the toughest metallic materials reported to date. The study highlights the importance of alloy composition and microstructure in achieving exceptional mechanical properties, and demonstrates that the nature of the alloying elements is more important than their sheer number. The CrCoNi alloy is compared to other materials, including high-entropy alloys and twinning-induced plasticity steels, and is found to have superior mechanical properties. The research provides insights into the mechanisms underlying the exceptional damage tolerance of the CrCoNi alloy at cryogenic temperatures.