In this paper we develop a detailed analysis of critical prewetting in the context of the two-dimensional Ising model. Namely, we consider a two-dimensional nearest-neighbor Ising model in a 2N·N rectangular box with a boundary condition inducing the coexistence of the + phase in the bulk and a layer of - phase along the bottom wall. The presence of an external magnetic field of intensity h=lambda/N (for some fixed lambda>0) makes the layer of - phase unstable. For any ß>ß_c, we prove that, under a diffusing scaling by N^{-2/3} horizontally and N^{-1/3} vertically, the interface separating the layer of unstable phase from the bulk phase weakly converges to an explicit Ferrari-Spohn diffusion.