N-6-methyladenosine (m(6)A), catalyzed by Mettl3 methyltransferase, is a highly conserved epigenetic modification in eukaryotic messenger RNA (mRNA). Previous studies have implicated m(6)A modification in multiple biological processes, but the in vivo function of m(6)A has been difficult to study, because mettl3 mutants are embryonic lethal in both mammals and plants. In this study, we have used transcription activator-like effector nucleases and generated viable zygotic mettl3 mutant, Zmettl3(m/m), in zebrafish. We find that the oocytes in Zmettl3(m/m) adult females are stalled in early development and the ratio of full-grown stage (FG) follicles is significantly lower than that of wild type. Human chorionic gonadotropin-induced ovarian germinal vesicle breakdown in vitro and the numbers of eggs ovulated in vivo are both decreased as well, while the defects of oocyte maturation can be rescued by sex hormone in vitro and in vivo. In Zmettl3(m/m) adult males, we find defects in sperm maturation and sperm motility is significantly reduced. Further study shows that 11-ketotestosterone (11-KT) and 17-estradiol (E2) levels are significantly decreased in Zmettl3(m/m), and defective gamete maturation is accompanied by decreased overall m(6)A modification levels and disrupted expression of genes critical for sex hormone synthesis and gonadotropin signaling in Zmettl3(m/m). Thus, our study provides the first in vivo evidence that loss of Mettl3 leads to failed gamete maturation and significantly reduced fertility in zebrafish. Mettl3 and m(6)A modifications are essential for optimal reproduction in vertebrates.