Distant metastasis of head and neck squamous cell carcinoma (SCC) to the infraclavicular lymph nodes-with the exception of the upper mediastinal lymph nodes-is rare. We report the case of a 44-year-old man who was treated with surgery and radiotherapy for SCC of the floor of the mouth. During regular follow-up 6 months after the cessation of radiotherapy, F18-fluorodeoxyglucose positron-emission tomography/computed tomography (FDG-PET/CT) detected a hypermetabolic lesion in the left lobe of the liver that was diagnosed as a metastasis of the head and neck SCC; no locoregional recurrence was found. The metastasis was surgically removed and more radiotherapy was administered, but the SCC recurred at the same site and the patient died of disseminated disease 12 months after the appearance of the first metastasis. To the best of our knowledge, this is the first reported case of a solitary peritoneal lymph node metastasis from an SCC of the floor of the mouth. We believe that regular FDG-PET/CT follow-up scans are useful for the detection of unusual distant metastases of head and neck cancers.