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Significance Androgen is essential for the masculinization of external genitalia such as the organ size and the male-type urethra in mammals. However, the genes downstream of androgen, which are responsible for these masculinization processes, have not been identified. Here, we show v-maf avian musculoaponeurotic fibrosarcoma oncogene homolog B ( Mafb ) as an essential masculinization gene for embryonic urethral formation. Mafb expression is prominent in developing male external genitalia, driving masculinization of embryonic urethral formation in an androgen-dependent manner. External genitalia of Mafb KO males exhibit urethral defects, giving insight into human hypospadias. The current findings indicate that Mafb is a crucial mediator of urethral masculinization and is a possible new candidate gene for hypospadias derived from embryonic abnormalities. Masculinization of external genitalia is an essential process in the formation of the male reproductive system. Prominent characteristics of this masculinization are the organ size and the sexual differentiation of the urethra. Although androgen is a pivotal inducer of the masculinization, the regulatory mechanism under the control of androgen is still unknown. Here, we address this longstanding question about how androgen induces masculinization of the embryonic external genitalia through the identification of the v-maf avian musculoaponeurotic fibrosarcoma oncogene homolog B (Mafb) gene. Mafb is expressed prominently in the mesenchyme of male genital tubercle (GT), the anlage of external genitalia. MAFB expression is rarely detected in the mesenchyme of female GTs. However, exposure to exogenous androgen induces its mesenchymal expression in female GTs. Furthermore, MAFB expression is prominently down-regulated in male GTs of androgen receptor ( Ar ) KO mice, indicating that AR signaling is necessary for its expression. It is revealed that Mafb KO male GTs exhibit defective embryonic urethral formation, giving insight into the common human congenital anomaly hypospadias. However, the size of Mafb KO male GTs is similar with that of wild-type males. Moreover, androgen treatment fails to induce urethral masculinization of the GTs in Mafb KO mice. The current results provide evidence that Mafb is an androgen-inducible, sexually dimorphic regulator of embryonic urethral masculinization.