We present a rare case of retroperitoneal dedifferentiated liposarcoma (DDLPS) masquerading as rhabdomyosarcoma. The patient was a 74-year-old man, complaining a loss of appetite. Abdominal computed tomography revealed a retroperitoneal mass, 10 cm in diameter, between the liver and the right adrenal gland. The tumor was resected and histologically diagnosed as conventional DDLPS, in which dedifferentiated component was highly cellular and composed of pleomorphic anaplastic cells. After 3 years, the tumor recurred in the right retroperitoneal space. The recurrent tumor consisted of 2 components: lipogenic and nonlipogenic. The latter differ from the dedifferentiated component of the primary tumor. The tumor cells were small, round to ovoid cells with monomorphous, round, hyperchromatic nuclei, and scant cytoplasm. Interestingly, they were diffusely positive for myogenin and desmin. To rule out the possibility of the second primary, we performed fluorescence in situ hybridization to detect FOXO1 rearrangement. We failed to demonstrate splits of the probes. In contrast, high-level amplification of MDM2 was detected by dual-color in situ hybridization. Given the morphologic and molecular findings, the neoplasm was identified as a peculiar DDLPS mimicking rhabdomyosarcoma. Retroperitoneal rhabdomyosarcoma-like tumors of adults, therefore, should be distinguished carefully from DDLPS. It could be challenging when lipogenic component was absent, but in situ molecular analyses can be helpful.