Introduction: Adnexal masses in pregnancy are identified at a rate of 2 to 20 in 1000, which mostly are benign simple cysts. Although the majority of adnexal masses will resolve spontaneously by the beginning of the second trimester, some cases are persistent forms which can result in complications for both mother and fetus. Presentation of case: A 31-year-old pregnant woman was diagnosed with a left ovarian mucinous cystadenoma. The ovarian cyst was first detected in the 8th week of pregnancy, measured 5×5 cm and reached 40×30 cm at 38 weeks of gestation. The patient underwent an elective cesarean delivery along with left salpingo-oophorectomy at term. An alive female baby of 3000 grams was delivered with good APGAR scores at the first and fifth minutes. Histologic examination confirmed the diagnosis of a benign mucinous cystadenoma. Dicussion: The option of surgical intervention versus conservative management should be tailored to the patient’s physical symptoms, characteristics of tumor, and gestational age. An elective cesarean section along with salpingo-oophorectomy can be adopted for a giant ovarian cyst complicating term pregnancy. There is a risk of recurrence after a removal of ovarian mucinous cystadenoma. Conclusion: Ovarian cysts coexisting with pregnancy should be followed up properly by routine ultrasound and regular antenatal care. The decision to postpone surgical management of a complex mass until the time of delivery must balance the risks and benefits for mother and fetus. After surgery, the patient needs to be provided an appropriate follow-up care to manage postoperative recurrence.