BACKGROUND: Against the background of linguistic and cultural differences, there is a need for translation and adaptation from the English version of the Fugl-Meyer Assessment (FMA) to Japanese. In addition, there is no study of inter-rater reliability of FMA all domains for affected upper extremities with appropriate sample size based on the intraclass correlation coefficient (ICC) focusing on non-simultaneous assessment. OBJECTIVE: This study aimed (1) to translate the English version of the FMA and its administration/scoring manual; and (2) to investigate the psychometric properties of the Japanese version of the FMA in patients with stroke. METHODS: A prospective single-center study involving 30 patients was conducted. The FMA and the Action Research Arm Test, the Box-and-Block Test, and the Motor Activity Log were employed. The inter-rater/intra-rater reliability, the internal consistency, the validity, and the floor/ceiling effects were assessed. RESULTS: Regarding the non-simultaneous and simultaneous inter-rater reliability, ICC ranged from 0.809-0.983 (P<0.001) and 0.991-0.999 (P<0.001), respectively. Regarding the simultaneous intra-rater reliability, ICC ranged from 0.994-0.999 (P<0.001). The Cronbach's alpha was 0.973 in the non-simultaneous evaluation and 0.981 in the simultaneous evaluation. Regarding the validity, Spearman's rhos were higher than 0.92 for the FMA all domains and motor domain. The patients who showed the highest score and the lowest score of the FMA (all domains and motor domain) were 10% and 0%, respectively. CONCLUSIONS: The Japanese version of the FMA motor domain and all domains can reliably assess the affected upper extremities in patients with mild-to-severe hemiparesis after stroke for both non-simultaneous and simultaneous assessment.