BACKGROUND Retroperitoneal benign cysts during pregnancy are extremely rare and often remain asymptomatic until they attain a very large size. Diagnosis typically relies on a pathological tissue biopsy. The decision to pursue 1-step or 2-step surgical treatment should be tailored to each individual case rather than generalized. METHODS This case report presents the unique scenario of a pregnant woman with a confirmed pregnancy complicated by a large retroperitoneal cyst. The patient had a retroperitoneal cyst during her initial pregnancy, which went undetected during the first cesarean section. However, it was identified during her second pregnancy by which time it had grown to 13.0 cm × 15.0 cm × 25.0 cm, and extended from the liver margin to right ovarian pelvic infundibulopelvic ligament. Consequently, it was removed smoothly during her second cesarean section. METHODS Postoperative pathology results indicated a massive retroperitoneal mucinous cystadenoma. METHODS The giant retroperitoneal cyst was smoothly excised during the second cesarean delivery for 1-step surgical treatment. RESULTS Under the combined spinal and epidural anesthesia, a live female infant was delivered at 38 3/7 gestational weeks and the neonatal weight was 3200g. Under general anesthesia with endotracheal intubation, the giant retroperitoneal cyst was excised smoothly without complications. CONCLUSIONS The findings of this case report contribute to the understanding of the diagnostic modalities, surgical approaches and postoperative considerations of giant retroperitoneal cysts associated with pregnancy.