SRY is the master regulator of male sex determination in eutherian mammals. In mice, Sry expression is transcriptionally and epigenetically controlled in a developmental stage-specific manner. The Sry promoter undergoes demethylation in embryonic gonadal somatic cells at the sex-determining period. However, its molecular mechanism and in vivo significance remain unclear. Here, we report that the Sry promoter is actively demethylated during gonadal development, and TET2 plays a fundamental role in Sry demethylation. Tet2-deficient mice showed absence of 5-hydroxymethylcytosine in the Sry promoter. Furthermore, Tet2 deficiency diminished Sry expression, indicating that TET2-mediated DNA demethylation regulates Sry expression positively. We previously showed that the deficiency of the H3K9 demethylase Jmjd1a compromises Sry expression and induces male-to-female sex reversal. Tet2 deficiency enhanced the sex reversal phenotype of Jmjd1a-deficient mice. Thus, TET2-mediated active DNA demethylation and JMJD1A-mediated H3K9 demethylation contribute synergistically to sex determination.