A 56-year-old man was referred with lower rectal cancer showing anal canal invasion and liver metastasis. He underwent an abdominoperineal resection and a partial hepatectomy. Adjuvant therapy with tegafur-uracil and leucovorin was administered postoperatively. Lung metastasis was detected 2 years later and was resected. Right mandibular metastasis was diagnosed 2 months after the resection of the lung metastasis. A partial mandibular resection was performed after chemoradiotherapy, followed by reconstruction with a titanium frame and oral cavity reconstruction with a greater pectoral musculocutaneous flap. The pathological diagnosis was metastatic rectal cancer, and the therapeutic effect chemoradiotherapy was Grade 2. He is presently alive without any evidence of cancer, and has maintained a good quality of life 3 years after the mandibular resection and more than 5 years after his first operation. Mandibular metastasis from rectal cancer is very rare and the prognosis is poor according to the literature, so this case is considered to be very unusual.