Here, we report an unusual case of sarcomatoid carcinoma mimicking extraskeletal osteosarcoma that manifested as recurrent rectal cancer. Five years earlier, a 76-year-old male patient had undergone neoadjuvant chemoradiotherapy followed by a laparoscopic low anterior resection due to adenocarcinoma of the rectum. He was admitted because of pain in the anus and left hip. He underwent abdominal computed tomography that revealed a newly developed left perirectal mass with gluteus maximus invasion measuring up to 8 cm, and therefore, an abdominoperineal resection was performed. Histologically, the tumor revealed sheets of spindled or epithelioid cells, an absence of gland formation, mucicarmine and periodic acid–Schiff stain negativity, and prominent intercellular deposits of osteoid-like calcified tissue. Tumor cells were diffusely immunoreactive for vimentin and cytokeratins. Ultrastructural examination demonstrated microvilli on the surface or within intercellular spaces. In this report, we also discuss the possible pathogenesis as well as the differential diagnosis.