Light is the key entraining stimulus for the circadian clock, but several features of the signaling pathways that convert the photic signal to clock entrainment remain to be deciphered. Here, we show that light induces the production of hydrogen peroxide (H₂O₂) that acts as the second messenger coupling photoreception to the zebrafish circadian clock. Treatment of light-responsive Z3 cells with H₂O₂ triggers the induction of zCry1a and zPer2 genes and the subsequent circadian oscillation of zPer1. Remarkably, the induction kinetics and oscillation profile in response to H₂O₂ are identical to those initiated by light. Catalase (Cat), an antioxidant enzyme degrading H₂O₂, shows an oscillating pattern of expression and activity, antiphasic to zCry1a and zPer2. Interestingly, overexpression of zCAT results in a reduced light-dependent zCry1a and zPer2 gene induction. In contrast, inhibition of zCAT function enhances light-mediated inducibility of these clock genes. These findings implicate the enzymatic function of zCAT enzyme in the negative regulation of light-dependent clock gene transcriptional activation. Our findings provide an attractive link between the regulation of the cellular reduction/oxidation (redox) state and the photic signaling pathways implicated in circadian control.