In this letter, we investigate the physical layer security of a mixed visible light communication (VLC)/radio-frequency (RF) system. More specifically, the relay can extract the direct current component and collect energy from the optical signal in the VLC link and then use this collected energy to retransmit data though the RF link. With this setup, the exact performance expressions for secure outage probability and average secrecy capacity are derived in closed form. We also provide asymptotic results to investigate the effect of different system parameters on the system performance. Finally, we present numerical results to illustrate our analytical result.