The study aimed to test differences in the hormones of the hypothalamus-pituitary-gonad axis, Kisspeptin (Kiss) and sex steroids, and the sexual maturation of goldfish (Carassius auratus) according to various photoperiods: 10 h light (L):14 h dark (D) (short day), 12L:12D (control), and 14L:10D (long day). To investigate the sexual maturation under various photoperiods, quantitative real-time PCR (QPCR) assays for salmon gonadotropin-releasing hormone, chicken gonadotropin-releasing hormone, gonadotropin hormones (GTHs), Kisspeptin 1 (Kiss1), and its receptor, G protein-coupled receptor 54 (GPR54), were developed. mRNA expression was monitored in cultured pituitary cells (in vitro) of various photoperiod groups. Furthermore, we injected or treated the animals with Kiss and found that Kiss treatment increases the mRNA expression levels of GTHs. We measured the plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH), and $17{alpha}$-hydroxypregnenolone levels. The gonadotropin-releasing hormones (GnRHs), GTHs, Kiss1/GPR54 mRNA expression, plasma FSH, LH, and $17{alpha}$-hydroxypregnenolone levels in the 14L:10D group were significantly higher than in the other groups. These results suggest that light length regulates sex maturation by GnRH and Kiss1 in the brain of the goldfish.