The role of growth twin boundaries on the slip activity of gold nanopillars under uniaxial compression was investigated by molecular dynamics simulation. A new type of size-dependent strengthening was found in twinned gold nanopillars. Strengthening resulted from slip arrests in the form of Lomer−Cottrell locks at the intersection of partial dislocations and twin boundaries. The significance of such phenomenon was found to depend on the twin size. These findings could help engineers design tunable mechanical properties in nanowires by interfacial plasticity.