Despite numerous technological applications associated to nickel silicide thin films, their formation mechanisms are still far from being understood. We combined experimental and numerical approaches to unravel the early stages of nickel silicide formation with an atomistic precision. In particular, we employed first principles calculations, X-ray reflectivity as well as high-resolution scanning transmission electron microscopy analyses. Altogether, our work demonstrates that during nickel deposition on top of a silicon surface, an interface alloyed layer is formed even at room temperature before any thermal activation. Moreover, we managed to determine that this interfacial layer has a nickel-rich Ni3Si composition which is favored by the ability of nickel atoms to penetrate the surface layers of the silicon substrate.