Objective General fatigue is one of the most frequent chief complaints in primary care, and an accurate assessment of fatigue has a direct impact on a patient's quality of life and treatment decisions. The Fatigue Assessment Scale (FAS), a measure of general fatigue, is useful for assessing fatigue in diverse cultures and diseases. However, there has been no study showing the reliability and validity of the scale in the Japanese context. The present study assessed the reliability and validity of the Japanese version of the FAS. Methods This study was conducted on 649 patients with long COVID who had a high frequency of general fatigue. To test the structural validity of the FAS, the patients were randomly divided into two groups: one in which an exploratory factor analysis (EFA) was conducted and one in which a confirmatory factor analysis (CFA) was conducted. Cronbach's alpha was calculated to assess internal consistency reliability. Results As 58 patients had missing values, we analyzed the data of 591 patients. The EFA led to an FAS comprising two factors. The CFA showed an acceptable fit for this two-factor model. The internal consistency was found to be good (Cronbach's alpha =0.89). Conclusion This study verified the structural validity and internal consistency and reliability of the Japanese version of the FAS. The results indicate that the Japanese version of the FAS is useful for assessing general fatigue in patients with long COVID in Japan.