The study investigates the dislocation nucleation-controlled softening mechanism in nano-twinned metals, where dislocation nucleation sites are abundant but dislocation motion is not confined. This mechanism governs the strength of such materials, leading to softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation show that there is a transition in deformation mechanisms at a critical twin-boundary spacing, where strength is maximized. The transition occurs when dislocation nucleation and motion parallel to twin planes become dominant, switching from the classical Hall–Petch strengthening to dislocation-nucleation-controlled softening. The critical twin-boundary spacing for this transition depends on the grain size, with smaller grains exhibiting higher maximum strength. The findings are supported by simulations of nano-twinned ultrafine-grained copper, which show that the strength first increases with decreasing twin-boundary spacing and then decreases further. The study provides insights into the strength softening in nano-twinned metals, highlighting the importance of dislocation nucleation at grain boundary-twin intersections.The study investigates the dislocation nucleation-controlled softening mechanism in nano-twinned metals, where dislocation nucleation sites are abundant but dislocation motion is not confined. This mechanism governs the strength of such materials, leading to softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation show that there is a transition in deformation mechanisms at a critical twin-boundary spacing, where strength is maximized. The transition occurs when dislocation nucleation and motion parallel to twin planes become dominant, switching from the classical Hall–Petch strengthening to dislocation-nucleation-controlled softening. The critical twin-boundary spacing for this transition depends on the grain size, with smaller grains exhibiting higher maximum strength. The findings are supported by simulations of nano-twinned ultrafine-grained copper, which show that the strength first increases with decreasing twin-boundary spacing and then decreases further. The study provides insights into the strength softening in nano-twinned metals, highlighting the importance of dislocation nucleation at grain boundary-twin intersections.