Kisspeptins have emerged as potent regulators of the reproductive brain-pituitary-gonad (BPG) axis. Our previous study demonstrated that the brain of the chub mackerel (Scomber japonicus), a scombroid fish, expresses two kisspeptin-encoding genes, kiss1 and kiss2, and exhibits sexually dimorphic expression profiles. Recent studies strongly suggest that teleost Kiss1 and Kiss2 precursors produce mature Kiss1-pentadecapeptides (Kiss1–15) and Kiss2-dodecapeptides (Kiss2–12), respectively. In light of the above, the present study evaluated the potency of synthetic peptides of Kiss1–15, Kiss2–12, and a GnRH analog (GnRHa) on inducing gonadal development in sexually immature adult chub mackerel. Synthetic peptides were administered subcutaneously through mini-osmotic pumps. On day 45 post-administration, gonadosomatic index (GSI) values (%) of male fish treated with Kiss1–15 (1.82) significantly increased in comparison to initial control (0.33), final control (0.49), Kiss2–12 (0.24), and GnRHa (1.13)-treated fish. Interestingly, the testis of all Kiss1–15 treated fish revealed spermiation, and were full of spermatozoa. These fish showed significantly higher levels of pituitary fshβ and Ihβ mRNAs and circulating 11-ketotestosterone. GnRHa treated fish also revealed the presence of few spermatozoa in the testis. In females, no significant changes in GSI values were found between treatments; however, Kiss1–15- and GnRHa-treated fish showed prominent signs of vitellogenic onset, with many early yolk oocytes in their ovaries. Interestingly, Kiss1–15-treated fish exhibited higher levels of pituitary fshβ and circulating estradiol-17β. These results indicate that peripheral administration of Kiss1–15 and GnRHa can induce gonadal development in sexually immature chub mackerel.