Adult granulosa cell tumours (AGCTs) of the ovary are very rare during pregnancy. To date, only five cases of ovarian AGCT in pregnancy have been reported in the literature and the patients all conceived spontaneously. We report a case of AGCT of the ovary that was incidentally discovered during a caesarean section in a patient undergoing In vitro fertilisation (IVF). To the authors’ knowledge, this is the first case of AGCT incidentally discovered during caesarean section in a pregnant patient after IVF. A 44-year-old primigravida with 39 weeks gestation was admitted to our hospital due to premature rupture of membranes in May 2019. She was treated by in vitro fertilisation due to being an elderly mother and she was pregnant after the first cycle. She was indicated for caesarean section due to conceiving following in vitro fertilisation and being an elderly mother. She gave birth to a 3,000 g baby boy and his Apgar scores were 8/1’–9/5’. When examining the adnexa, the left ovary had a tumour with a size of 7 × 4 × 4 cm. Left oophorectomy was performed and specimen sent to for histopathology. The histopathological diagnosis was an AGCT of the ovary. A month later, the patient received chemotherapy with Carboplatin and Paclitaxel for four cycles. After 32 months of follow-up, no recurrence was detected. In conclusion, AGCTs of the ovary are very rare during pregnancy. Pre-operative diagnosis is difficult. Conservative surgery should be considered in women who wish to have children. Patients should receive adequate counselling and long-term follow-up to ensure the highest survival rates and early detection of recurrence.