Abstract Functional gonadotroph adenomas (FGAs) are rare, manifesting symptoms like menstrual irregularities or ovarian hyperstimulation syndrome (OHSS). We present a case of OHSS caused by an FGA during the follow-up of a pituitary tumor initially considered nonfunctioning. The patient presented with lower abdominal pain, abdominal swelling, and dyspnea. Magnetic resonance imaging (MRI) revealed 15 cm enlarged ovarian cysts and pleural effusion. Laboratory examination showed an elevated serum estradiol (E2) level (5741.4 pmol/L [1564.0 pg/mL]), suppressed luteinizing hormone, and nonsuppressed follicular-stimulating hormone (FSH). However, no pituitary hormone disorders were observed when a 19 mm pituitary tumor was discovered 11 months prior. Given the absence of human chorionic gonadotropin (hCG) administration, OHSS due to the FGA was suspected. Cabergoline, known for alleviating the severity of OHSS, was administered, but the ovarian cysts continued to enlarge. Subsequently, endoscopic transsphenoidal surgery was performed, and immunohistochemical analysis confirmed the diagnosis of the FSH-producing adenoma. Follow-up MRI scans showed reduced ovarian cysts and successful pituitary tumor resection with a reduced E2 level. This case highlights the importance of considering FGAs when encountering OHSS without hCG administration or following up on pituitary tumors in premenopausal female patients to take appropriate measures for accurate diagnosis and management.