<p>Making ultrahigh-strength as-quenched carbon martensitic steels ductile remains a critical challenge for structural applications. The ordered occupancy of carbon at interstitial sites in body-centred cubic martensite induces anisotropic lattice distortion, forming brittle body-centred tetragonal martensite with suppressed dislocation activity. Conventional tempering eliminates this distortion to improve ductility. Here we propose a counterintuitive strategy to unlock the ductility of a 2.4-GPa as-quenched carbon martensitic steel by utilizing the anisotropic lattice distortion of martensite. Its severe lattice distortion, that is, its high tetragonality, is driven by large-concentration substitutional solutes and carbon. The deliberately introduced high tetragonality activates deformation twins as a plastic carrier, effectively overcoming the brittleness of quenched carbon martensitic steel. This strategy of using solid-solution-induced anisotropic lattice distortion challenges the conventional view of tetragonal martensite’s inherent brittleness, establishing a framework for alloy design that yields strong and ductile metallic materials.</p>