Trousseau syndrome is a clinical condition in which thromboembolism is caused by hypercoagulation associated with a malignant tumor. We report a case of cancer of the floor of the mouth with Trousseau syndrome. A 73-year-old man was referred to our hospital for cancer of the floor of the mouth. A blood examination revealed a high level of D-dimer (10.0μg/mL) and ultrasonography showed deep vein thrombosis. Magnetic resonance imaging (MRI) also revealed acute multiple cerebral infarction, and we made a diagnosis Trousseau syndrome. PET-CT showed FDG accumulation in the right floor of the mouth and the right submandibular lymph nodes, and in multiple bones. Based on the diagnosis of Trousseau syndrome, continuous intravenous administration of heparin was performed. At the end of the palliative irradiation for lumbar vertebrae, bleeding from the primary cancer occurred and heparin had to be discontinued. He was then transferred to the referring hospital for supportive care. He died from multiple organ failure about 2 months after first visiting our hospital.