Y box-binding protein 1 (Ybx1/ybx1) regulates gene expression through DNA/RNA binding. In zebrafish, Ybx1 is highly abundant in primary growth (PG) follicles in the ovary, but decreases precipitously as the follicles enter the secondary growth (SG). To understand Ybx1 function in folliculogenesis, we created a ybx1 mutant using TALEN and observed disrupted folliculogenesis during the previtellogenic (PV) to early vitellogenic (EV) transition of SG, resulting in underdeveloped ovaries and infertility. Expression and western blot analyses revealed differential gene expression between ybx1^−/− and control ovaries, with significantly increased expression of cdkn1a (p21), a cell cycle inhibitor, in ybx1^−/− follicles. While cdkn1a knockout via CRISPR/Cas9 was embryonically lethal, the heterozygote (cdkn1a^+/−) displayed advanced follicle activation and maturation, contrasting with the ybx1^−/− phenotype. Partial loss of p21 alleviated the ybx1^−/− phenotype, restoring folliculogenesis with normal PG-PV and PV-EV transitions in ybx1^−/−;cdkn1a^+/− mutants. While ybx1^−/− mutant follicle cells displayed poor proliferation in vivo and in vitro, the cells from the ybx1^−/−;cdkn1a^+/− follicles resumed normal proliferation. In conclusion, Ybx1 is crucial for early folliculogenesis in zebrafish, potentially by repressing cdkn1a expression, either directly or indirectly.