Brenner tumors are extremely rare epithelial ovarian neoplasms that are often asymptomatic and difficult to diagnose during pregnancy. Only four cases have been reported in the literature. Here, we describe the case of a 30-year-old pregnant woman with a large (26 × 20 × 8 cm^3), left-side, ovarian, borderline stage Ia Brenner tumor complicated with torsion during the first trimester. This patient successfully underwent unilateral salpingo-oophorectomy during the first trimester, to eventually deliver a term male baby without any complications.