Abstract: We report the experimental observation of light induced terahertz surface plasmons. By use of an optical pump-terahertz probe technique, an instantaneous transition is induced between two interesting optical phenomena, an out-of-plane photonic crystal effect and surface-plasmon resonance, in a thin silicon film perforated with an array of subwavelength holes. The dielectric function of silicon is essentially altered by the intense ultrafast laser pulses due to photo-generated free carriers, whereby the silicon array becomes metallic and favors the coupling and propagation of surface plasmons in the terahertz regime. The plasmonic resonances are well fitted by the Fano model.