We report a two-dimensional platinum monolayer (Pt-ML) sheet for oxygen reduction reaction (ORR) activity using first-principles calculations. Unlike previous reports of a supported hexagonal planar monolayer, the Pt-ML exhibits an orthorhombic buckled structure, where each Pt atom is coordinated with six Pt atoms. State-of-the-art calculations show that the Pt-ML is energetically, thermally, dynamically and mechanically stable and thus can be synthesized. An orbital mixing between an in-plane σ-orbital and out-of-plane π-orbital helps in stabilizing the buckling pattern. We find that the dz2 orbitals of the out-of-plane Pt atoms tilt themselves (by 30°) toward the dyz orbital of the in-plane Pt atoms to gain the maximum overlap, which in turn stabilizes the buckled structure. The potential applicability of the Pt-ML towards ORR activity has been investigated and we find that the ORR rate determining step (OH formation) is significantly improved when catalyzed by the Pt-ML compared to any other catalysts reported to date. Our potential dependent study shows that the ORR is thermodynamically favourable at 0.38 V and thus lowers the overpotential for the ORR. Besides, the Pt-ML is much more selective towards H2O formation over H2O2 formation. © The Royal Society of Chemistry.