The golden rabbitfish Siganus guttatus is a reef fish with a restricted lunar-synchronized spawning cycle. It is not known how the fish recognizes cues from the moon and exerts moon-related activities. In order to evaluate the perception and utilization of moonlight by the fish, the present study aimed to clone and characterize Period2 (Per2), a light-inducible clock gene in lower vertebrates, and to examine daily variations in rabbitfish Per2 (rfPer2) expression as well as the effect of light and moonlight on its expression in the pineal gland. The partially-cloned rfPer2 cDNA (2933 bp) was highly homologous (72%) to zebrafish Per2. The rfPer2 levels increased at ZT6 and decreased at ZT18 in the whole brain and several peripheral organs. The rfPer2 expression in the pineal gland exhibited a daily variation with an increase during daytime. Exposing the fish to light during nighttime resulted in a rapid increase of its expression in the pineal gland, while the level was decreased by intercepting light during daytime. Two hours after exposing the fish to moonlight at the full moon period, the rfPer2 expression was upregulated. These results suggest that rfPer2 is a light-inducible clock gene and that its expression is affected not only by daylight but also by moonlight. Since the rfPer2 expression level during the full moon period was higher than that during the new moon period, the monthly variation in the rfPer2 expression is likely to occur with the change in amplitude between the full and new moon periods.