Pt–Sn alloys play an important role as oxygen reduction reaction (ORR) catalysts in fuel cell technology both for their role in rationalizing the mechanism responsible for the better performances of bimetallic catalysts and for their practical applications. Here we present a complete experimental and theoretical study on the geometric and electronic structure of a (4×1) termination over a Pt3Sn near-surface ordered alloy obtained by depositing Sn on the Pt(110) and subsequent annealing. LEED and STM measurements combined with DFT simulations, allow us to determine the atomistic structure of this phase, and to rationalize the peculiar dependence of its STM pattern on the applied bias and the compositional order in deep layers. Both photoemission experiments and density functional calculations indicate that in this phase the Sn-Pt alloying process determines important changes in the electronic properties, and in particular a relevant shift of the Pt 5d band centroid with respect to clean Pt.