Heusler Ni–Mn–(Ga, In, Sn, Sb) materials can provide large magnetic-field-induced strain, giant magnetocaloric and magnetoresistance effects based on their first-order solid-state martensitic transformation. In the present work, effects of Co doping on martensitic transformation behavior in melt-spun Ni–Mn–Sn ribbons were studied by x-ray diffraction, scanning/transmission electron microscopy, and thermal analysis. Experimental results showed that both martensitic transition and austenite Curie temperatures increased linearly with Co addition to Ni49Mn39Sn12; and meanwhile, crystal structures of the martensite evolved from four-layered orthorhombic (4O) to five-layered orthorhombic (10M), and then seven-layered monoclinic (14M). The compositional dependence of the martensitic transition temperatures was well correlated with changes of valence electron concentration (e/a) and unit-cell volume of high-temperature austenite. It was proposed that both increase of valence electron concentration and shrinkage of austenite unit-cell volume with Co addition are favorable to the occurrence of martensitic transformation. In addition, the Curie temperature of austenite increases with Co addition, which was ascribed to the enhancement of ferromagnetic exchange interaction.