The optoelectronic properties of a monolithically integrated series-connected tandem solar cell are simulated using a drift-diffusion model. Following the large success of hybrid organic-inorganic perovskites for photovoltaics, which have recently demonstrated large efficiencies with low production costs1,2, we examine the possibility of using the same perovskites as absorbers in a tandem solar cell. The cell consists in a methyl ammonium mixed bromide-iodide lead perovskite, CH3NH3PbI3(1-x)Br3x (0 ≤ x ≤ 1), top sub-cell and a single-crystalline silicon bottom sub-cell. A Si-based tunnel junction connects the two sub-cells. Numerical simulations are based on a one-dimensional numerical drift-diffusion model3. It is shown that a top cell absorbing material with 20% of bromide and a thickness in the 300-400 nm range affords current matching with the silicon bottom cell. Good interconnection between single cells is ensured by standard n and p doping of the silicon at 5.1019cm-3 in the tunnel junction. A maximum efficiency of about 27% is predicted for the tandem cell exceeding both the efficiencies of stand-alone silicon and perovskite cells taken for our simulations, which amount to 17.3% and 17.9%, respectively.