Post-radiation soft tissue sarcomas are recognized as rare complications of radiation therapy. The most common type of post-radiation soft tissue sarcoma is a malignant fibrous histiocytoma (MFH), which originates from mesenchymal cells with a predominance of histiocytes and fibroblasts. The two most common sites of occurrence for post-radiation soft tissue sarcomas are the chest wall and pelvic cavity. Post-radiation colorectal MFHs are extremely rare and all of the reported cases of post-radiation sarcomas have occurred >3 years after radiation therapy. Recently, we managed a case of colorectal MFH which developed in a 48-year-old male who had undergone a low anterior resection for rectal adenocarcinoma and had received chemoradiotherapy as adjuvant treatment. Twelve months after radiotherapy, a 4 cm mass was detected 8 cm superior to the anastomosis site on colonoscopic examination. A soft tissue sarcoma was suspected on pathologic examination of the biopsy specimen. Therefore, he underwent a Hartmann's operation and the final pathologic finding revealed MFH with a storiform pattern of tumor cells composed of pleomorphic, multinucleated giant cells. This is the first case of MFH that had a latency period <3 years (i.e., 1 year) between the time of radiotherapy and diagnosis.