Significance Sex development involves a precise spatiotemporal expression and interactions of numerous genetic factors, including the WT1 (Wilms tumor 1) gene. Complete and partial loss-of-function WT1 variants are associated with 46,XY disorders/differences of sex development (DSD). Some 46,XX individuals develop testis in absence of the testis-determining gene SRY . We describe a genotype/phenotype association where variants impacting the C-terminal zinc finger (ZF4) of WT1 cause testis development in 46,XX individuals. XX mice carrying a pathogenic variant of ZF4 display masculinization of the fetal gonads. Testis formation may be due to inappropriate interaction between the mutated WT1 and an essential ovarian determinant β-CATENIN. These data show that variants affecting a specific domain of a developmental transcription factor can switch organ fate.