Fatigue has been recognized as one of the most distressing symptoms in cancer patients. Concise assessment is essential to managing this symptom. To that end, the Brief Fatigue Inventory (BFI), a 9-item questionnaire, was designed to assess fatigue in cancer patients. The purpose of this study was to examine the validity and reliability of the Japanese version of this scale (BFI-J), when compared with previously validated fatigue instruments. We randomly selected 252 cancer patients and presented them with the BFI-J, along with the Cancer Fatigue Scale; Profile of Mood States fatigue, vigor, and depression subscales; and European Organization for Research and Treatment of Cancer QLQ-C30. Specifically, the reliability and construct, criterion, convergent, and discriminant validity of each instrument were evaluated. Additionally, fatigue severity classification was explored using the BFI-J. The results indicated that the BFI-J is a brief, valid, and feasible measure of fatigue for use with Japanese cancer patients.