We report on the proof of principle of a III-nitride/silicon tandem solar cell. Photovoltaic activity is demonstrated in a 0.25 cm(2) dual junction solar cell, made of p- and n-type GaN layers which were grown by molecular beam epitaxy (MBE) on a standard n-type Si wafer with an Al doped p-type surface. An open circuit voltage (V(oc)) of 2.4 V was measured under 1 x sun AM1.5G condition with additional UV laser illumination of the GaN junction. Experiments under various illumination conditions were performed to verify that both junctions are active and working in series. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.122202