We report the formation of deformation twins in a nanocrystalline Cu–25 at.% Ni alloy with medium stacking fault energy via the propagation of Σ3{1 1 2} incoherent twin boundaries (ITBs) that are nucleated from grain boundaries. This deformation twinning process does not generate net macroscopic strain since the Burgers vectors of the three partial dislocations on three successive {1 1 1} planes comprising an ITB sum to zero. The moving direction of ITBs determines the occurrence of twinning or detwinning in nanocrystalline materials.