Primary retroperitoneal cystic lesions (PRCLs) are extremely rare, and their histogenesis and clinical evolution remain unclear. Here, we report the case of a 38-years-old primigravida who was referred to our hospital for the delivery. Routine ultrasonographic examination at 36 weeks of gestation revealed the presence of a cystic mass close to the right side of the uterus. One month after the childbirth, we performed the surgery. The cystic mass was present on the right side of the retroperitoneal cavity. We successfully extracted the cyst without rupture. The cyst was 12 × 6 × 3 cm, unilocular, exhibited a thin wall, and contained mucinous liquid. Histological examination revealed that the lining of the cyst comprised two types of adjacent cells. The flat low cuboidal cells were positive for calretinin, cytokeratin (CK) 5/6, CK7, cancer antigen 125 (CA125), and D2-40, all of which are mesothelial markers. The tall columnar cells were also positive for the mesothelial marker CK7. However, the flat low cuboidal cells were negative for mucinous cell makers, while tall columnar cells were positive for these markers. The tall columnar cells contained mucin but were not positive for gastrointestinal epithelial-specific secreted mucin makers. This finding directly supports our hypothesis that the retroperitoneal cysts emerged from the mesothelium.