A 26-year-old woman presented with massive ascites and an increased CA-125 level suggestive of ovarian cancer. Laparotomy revealed 5L of hemorrhagic ascites, bilateral ovarian cysts, and areas of reddish color within the pelvis consistent with endometriosis. Histopathology of biopsy specimens confirmed endometriosis. The patient received GnRH analogue treatment for 6 months, experienced a reduction in ascites, and had full remission after 2 years. This case report highlights that endometriosis should be considered as a potential cause of ascites in reproductive-aged women, as it can mimic ovarian cancer.