Mucinous carcinoma of the retroperitoneal origin is extremely rare. The existence of mural nodules in association with retroperitoneal mucinous carcinoma is an even rarer condition and indicates a worse prognosis. We present a case of primary retroperitoneal mucinous carcinoma with carcinosarcomatous mural nodules in a 27-year-old woman. We found a histological spectrum of mucinous tumors encompassing a mucinous borderline tumor, microinvasive carcinoma, and overt carcinoma with an expansile invasive pattern. The mural nodules had two morphological components. The sarcomatous component consisted of diffusely proliferating pleomorphic spindle or polygonal cells, while the carcinomatous component exhibited infiltrative glands showing a complex and cribriform architecture as well as distorted and poorly formed small glands. The carcinomatous component, comprising approximately 20% of the entire tumor volume of the mural nodules, was randomly distributed within the sarcomatous component. In a few areas, the carcinomatous component transformed and merged into the sarcomatous component. Immunostaining revealed a mutually exclusive pattern of expression of cytokeratin and vimentin in the carcinomatous and sarcomatous components, respectively, supporting the presence of a dual tumor cell population and confirming the diagnosis of carcinosarcoma. In summary, our case exhibited a histological spectrum of mucinous tumors and a metaplastic transformation from the carcinomatous to sarcomatous component in mural nodules. The immunostaining results of a mutually exclusive expression pattern of epithelial and mesenchymal markers confirmed the histological evidence of a dual population. Although rare, the specific histological features and immunophenotype are helpful in establishing the diagnosis of carcinosarcomatous mural nodules. Since the pathogenetic mechanism and treatment strategies for primary retroperitoneal mucinous carcinoma remain unclear, pathologists have an essential role to play in correctly evaluating the presence of mural nodules and determining their nature, to be later utilized to predict patients’ outcomes and provide appropriate treatment.