The grain size is a determinant microstructural feature to enable the activation of deformation twinning in hexagonal close‐packed (hcp) metals. Although deformation twinning is one of the most effective mechanisms for improving the strength–ductility trade‐off of structural alloys, its activation is reduced with decreasing grain size. This work reports the discovery of the activation of deformation twinning in a fine‐grained hcp microstructure by introducing ductile body‐centered cubic (bcc) nano‐layer interfaces. The fast solidification and cooling conditions of laser‐based additive manufacturing are exploited to obtain a fine microstructure that, coupled with an intensified intrinsic heat treatment, permits to generate the bcc nano‐layers. In situ high‐energy synchrotron X‐ray diffraction allows tracking the activation and evolution of mechanical twinning in real‐time. The findings obtained show the potential of ductile nano‐layering for the novel design of hcp damage tolerant materials with improved life spans.