Although high concentrations of 2-ethylhexyl diphenyl phosphate (EHDPP) have been detected in wild fish, its reproductive toxicity in fish remains unclear. In this study, we for the first time observed that EHDPP elicited androgen receptor (AR) antagonistic activity with a 50% inhibitory concentration of 37.5 μM. 2-Ethyl-5-hydroxyhexyl diphenyl phosphate was proved to be the dominant metabolite of EHDPP in Japanese medaka and elicited 3.1-fold stronger AR antagonistic activity than that of EHDPP. Medaka larvae (0-day post hatching) were exposed to EHDPP for 100 days, and intersex was observed in males from all exposure groups with significantly increased incidence (13.5-48.6%). 17β-E2 was promoted at 104 ng/L, and androgens were suppressed at 434 ng/L, which account for the intersex incidence in the high-exposure groups but do not explain the significant incidence of intersex in the 29.9 ng/L exposure group. The AR antagonistic activity of EHDPP and its metabolites must therefore play a key role in intersex incidence. EHDPP also significantly (p < 0.05) repressed reproductive behaviors of males in the 434 ng/L group and decreased fertility in high-exposure groups compared with the control. All the adverse outcomes were observed under environmentally relevant concentrations, implying that EHDPP poses an ecological risk for wild fish populations.