The Achilles’ heel of additively manufactured Ti-6Al-4V by selective laser melting (SLM) is its inferior mechanical properties compared with its wrought (forged) counterparts. Acicular α′ martensite resulted from rapid cooling by SLM is primarily responsible for high strength but inadequate tensile ductility achieved in the as-fabricated state. This study presents a solution to eliminating the adverse effect of the nonequilibrium α′ martensite. This is achieved by enabling in situ martensite decomposition into a novel ultrafine (200–300 nm) lamellar (α + β) microstructure via the selection of an array of processing variables including the layer thickness, energy density, and focal offset distance. The resulting tensile elongation reached 11.4% while the yield strength was kept above 1100 MPa. These properties compare favorably with those of mill-annealed Ti-6Al-4V consisting of globular α and β. The fatigue life of SLM-fabricated Ti-6Al-4V with an ultrafine lamellar (α + β) structure has approached that of the mill-annealed counterparts and is much superior to that of SLM-fabricated Ti-6Al-4V with α′ martensite.