A new route towards strong yet ductile metals via architecting heterogeneities in both structure and metastability is presented. Such heterogeneities are generated and manipulated in a standard stainless steel using a heterogeneous phase transformation (het-PT) strategy, in which focused laser patterning is applied to stimulate site-specific phase transformations. The het-PT processed steel contains periodically arranged stripes of strong martensite and ductile austenite with different grain sizes and metastability. The resulting structural heterogeneity leads to a desirable strain gradient and heterogeneous deformation-induced martensitic transformation during deformation, which effectively accommodate strain localization and enhance work hardening capability in the het-PT-processed steel. These unique features result in an enhanced balance between strength and ductility, outperforming both homogeneously ultrafine-grained and coarse-grained counterparts. The het-PT strategy is expected to be applicable to tailoring structural heterogeneity and metastability in other steels and metals.