Although important factors governing the meiosis have been reported in the embryonic ovary, meiosis in postnatal testis remains poorly understood. Herein, we first report that SRY-box 30 (Sox30) is an age-related and essential regulator of meiosis in the postnatal testis. Sox30-null mice exhibited uniquely impaired testis, presenting the abnormal arrest of germ-cell differentiation and irregular Leydig cell proliferation. In aged Sox30-null mice, the observed testicular impairments were more severe. Furthermore, the germ-cell arrest occurred at the stage of meiotic zygotene spermatocytes, which is strongly associated with critical regulators of meiosis (such as Cyp26b1, Stra8 and Rec8) and sex differentiation (such as Rspo1, Foxl2, Sox9, Wnt4 and Ctnnb1). Mechanistically, Sox30 can activate Stra8 and Rec8, and inhibit Cyp26b1 and Ctnnb1 by direct binding to their promoters. A different Sox30 domain required for regulating the activity of these gene promoters, providing a “fail-safe” mechanism for Sox30 to facilitate germ-cell differentiation. Indeed, retinoic acid levels were reduced owing to increased degradation following the elevation of Cyp26b1 in Sox30-null testes. Re-expression of Sox30 in Sox30-null mice successfully restored germ-cell meiosis, differentiation and Leydig cell proliferation. Moreover, the restoration of actual fertility appeared to improve over time. Consistently, Rec8 and Stra8 were reactivated, and Cyp26b1 and Ctnnb1 were reinhibited in the restored testes. In summary, Sox30 is necessary, sufficient and age-associated for germ-cell meiosis and differentiation in testes by direct regulating critical regulators. This study advances our understanding of the regulation of germ-cell meiosis and differentiation in the postnatal testis.