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A Co–Cr–Mo alloy with predominantly martensitic structure was prepared by selective laser melting (SLM), where the amount of martensite formed in the SLM specimens is much larger than in the cast samples which can be ascribed to the high density of internal defects such as dislocations and SFs. We observe a hierarchical microstructure spanning over different length scales, consisting of remelted zones and track core regions at the macro-scale, regions with elongated columnar grains and equiaxed grains at the micro-scale, martensitic laths at the submicron-scale, and a high density of dislocations and stacking faults at the nano- and atomic scale. The hierarchical microstructure leads to the outstanding tensile properties of the as-prepared SLM and SLM annealed samples (with yield strength, ultimate tensile strength and fracture strain of ~ 735 MPa, ~ 1211 MPa and ~ 12.8%, and ~ 893 MPa, ~ 1214 MPa and ~ 13.8%, respectively). The mechanisms determining the strength and ductility are discussed in terms of the hierarchical microstructure and internal defects. The cellular structure with a high density of dislocations along their boundaries and the martensitic structure are the reasons for strengthening, while the hierarchical microstructure helps to obtain appropriate ductility. Furthermore, martensitic transformation induced plastic bending of the residual FCC plates was confirmed, which is helpful to maintain the different plastic strain between the heterogeneous structure. The development of the martensitic structure in this Co–Cr–Mo alloy by SLM and its characteristic hierarchical microstructure are promising features for material design and application of high-strength materials with improved strength‒ductility tradeoff.