Follicle-stimulating hormone (Fsh) is thought to act early in the process of spermatogenesis; however, its action in fish has not yet been clearly established. In the present work, we analyzed the effects of recombinant Fsh in sea bass (Dicentrarchus labrax) spermatogenesis according to two different approaches: direct injection of recombinant single-chain Fsh hormone (scFSH) and injection of scFSH coding sequence. Both approaches were efficient in increasing plasma Fsh at 7 and 15 days, respectively, after injection. The Fsh increment caused a significant increase in plasma 11-ketotestosterone levels and induced dramatic changes at the testicular level. Fsh-treated groups showed an increase in germ cell proliferation at Day 7, and cysts of spermatocytes and spermatids were observed at the end of the experiment. After treatment with Fsh, a suppression in amh transcripts and an increase of lhr transcripts were detected at Day 7 and Day 15, respectively, and an increment in fshr expression became evident at Day 23. These results show that Fsh initiates germ cell proliferation, triggering spermatogenesis in sea bass via androgen production and regulation of spermatogenesis-related genes.