This study investigates the deformation mechanisms and strength behavior of nano-twinned metals, focusing on how dislocation nucleation governs their strength and softening. In conventional metals, dislocations multiply and interact with grain boundaries and obstacles, controlling strength. However, in nanostructured materials, dislocation multiplication is restricted by nanoscale geometries, leading to source-controlled plasticity. Nano-twinned metals, however, have abundant dislocation nucleation sites but not restricted dislocation motion. The strength of these materials is governed by dislocation nucleation, resulting in softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation show a transition in deformation mechanism at a critical twin-boundary spacing, where strength is maximized. At this point, the classical Hall–Petch strengthening mechanism switches to a dislocation-nucleation-controlled softening mechanism involving twin-boundary migration. Previous studies did not consider a sufficient range of twin thickness, missing this strength-softening regime. The simulations indicate that the critical twin-boundary spacing and maximum strength depend on grain size: smaller grain sizes result in smaller critical twin-boundary spacing and higher maximum strength. Ultrafine-grained Cu with nanoscale twins shows increased strength and ductility compared to conventional Cu. The strength of nanotwinned Cu first increases with decreasing twin-boundary spacing, reaching a maximum at λ = 15 nm, then decreases as λ is further reduced. This trend can be explained by the Hall–Petch effect, but the subsequent strength softening is intriguing. In nano-twinned Cu, the observed strength softening is not due to grain-boundary-associated mechanisms. Instead, it is attributed to dislocation nucleation at grain boundary–twin intersections. The simulations show that the strength softening in nano-twinned Cu is governed by dislocation nucleation at these intersections. This mechanism is rare because dislocations can easily multiply in sufficient space. However, in micro- and nano-pillars, dislocation nucleation and multiplication are hindered by reduced structure dimensions. The study used molecular dynamics simulations to investigate the effect of twin thickness on deformation mechanisms in nano-twinned Cu. The simulations showed that the strength softening is governed by dislocation nucleation at grain boundary–twin intersections. The results, combined with experimental observations, suggest that dislocation nucleation governs the observed strength softening below a critical twin-boundary spacing. A theory of strength softening in nano-twinned metals was formulated, considering the kinetics of dislocation nucleation and available source density. The theory shows that the strength of the material depends on twin-boundary spacing and grain size. The study concludes that dislocation-nucleation-controlled softening is a key mechanism in nano-twinned metals, with the critical twin-boundary spacing and maximum strength depending on grain size. The findings are applicable to nano-twinned face-centred cubic metals.This study investigates the deformation mechanisms and strength behavior of nano-twinned metals, focusing on how dislocation nucleation governs their strength and softening. In conventional metals, dislocations multiply and interact with grain boundaries and obstacles, controlling strength. However, in nanostructured materials, dislocation multiplication is restricted by nanoscale geometries, leading to source-controlled plasticity. Nano-twinned metals, however, have abundant dislocation nucleation sites but not restricted dislocation motion. The strength of these materials is governed by dislocation nucleation, resulting in softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation show a transition in deformation mechanism at a critical twin-boundary spacing, where strength is maximized. At this point, the classical Hall–Petch strengthening mechanism switches to a dislocation-nucleation-controlled softening mechanism involving twin-boundary migration. Previous studies did not consider a sufficient range of twin thickness, missing this strength-softening regime. The simulations indicate that the critical twin-boundary spacing and maximum strength depend on grain size: smaller grain sizes result in smaller critical twin-boundary spacing and higher maximum strength. Ultrafine-grained Cu with nanoscale twins shows increased strength and ductility compared to conventional Cu. The strength of nanotwinned Cu first increases with decreasing twin-boundary spacing, reaching a maximum at λ = 15 nm, then decreases as λ is further reduced. This trend can be explained by the Hall–Petch effect, but the subsequent strength softening is intriguing. In nano-twinned Cu, the observed strength softening is not due to grain-boundary-associated mechanisms. Instead, it is attributed to dislocation nucleation at grain boundary–twin intersections. The simulations show that the strength softening in nano-twinned Cu is governed by dislocation nucleation at these intersections. This mechanism is rare because dislocations can easily multiply in sufficient space. However, in micro- and nano-pillars, dislocation nucleation and multiplication are hindered by reduced structure dimensions. The study used molecular dynamics simulations to investigate the effect of twin thickness on deformation mechanisms in nano-twinned Cu. The simulations showed that the strength softening is governed by dislocation nucleation at grain boundary–twin intersections. The results, combined with experimental observations, suggest that dislocation nucleation governs the observed strength softening below a critical twin-boundary spacing. A theory of strength softening in nano-twinned metals was formulated, considering the kinetics of dislocation nucleation and available source density. The theory shows that the strength of the material depends on twin-boundary spacing and grain size. The study concludes that dislocation-nucleation-controlled softening is a key mechanism in nano-twinned metals, with the critical twin-boundary spacing and maximum strength depending on grain size. The findings are applicable to nano-twinned face-centred cubic metals.