In this paper we describe the luminescence properties of Si nanowires (NWs) prepared by a maskless synthesis technique, based on the Au-catalyzed wet etching of Si substrates by an aqueous solution of H_2O_2 and HF. A strong room temperature photoluminescence (PL), centered at about 690 nm, is observed when Si NWs are optically excited. The detailed analysis of the steady-state and time-resolved PL properties of the system as a function of aging, temperature and pump power allows to demonstrate that the emission is due to the radiative recombination of quantum confined excitons. These results open the route towards novel applications of Si NWs in photonics as efficient light sources.