Primary retroperitoneal mucinous cystadenocarcinoma (PRMCAC) is rare. Approximately 50 cases have been reported in the English literature, and 2 cases were associated with pregnancy. Here we report the third case of PRMCAC in a woman during pregnancy. The 29-year-old woman was asymptomatic when a routine ultrasound examination at the 28th week of gestation revealed a suspected right ovarian cyst. A caesarean section delivery was performed at the 38th week of gestation, during which a large tumor was found intra-operatively in the right-retroperitoneal space. Laparotomic tumor resection was performed 2 months later, and the PRMCAC diagnosis was made.