Spinocerebellar ataxia type 21 (SCA21) is a rare subtype of autosomal dominant cerebellar ataxias, which was first identified in a French family and has been reported almost exclusively in French ancestry so far. We here report the first Japanese family with SCA21, in which all affected members examined carried a heterozygous c.509C > T:p.Pro170Leu variant in TMEM240. Their clinical features were summarized as a slowly progressive ataxia of young-adult onset (5-48 years) associated with various degree of psychomotor retardation or cognitive impairment. The MR images revealed atrophy in the cerebellum, but not in the cerebrum or brainstem. These clinical findings were consistent with those in the original French families with SCA21. Neuropathological findings in one autopsied patient showed a prominent decrease of cerebellar Purkinje cells, but no specific abnormalities outside the cerebellum.