alpha-Synucleinopathies, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, are neurodegenerative disorders in which abnormal inclusions containing alpha-synuclein accumulate in selectively vulnerable neurons and glia. In this report, immunohistochemistry demonstrates ubiquitin in subsets of alpha-synuclein inclusions in dementia with Lewy bodies and multiple system atrophy. Biochemistry demonstrates that alpha-synuclein in the sodium dodecyl sulfate-soluble fractions of diseased brains is ubiquitinated, with mono- and di-ubiquitinated species predominating over polyubiquitinated forms. Similar immunohistochemical and biochemical characteristics were observed in an A53T mutant human alpha-synuclein transgenic mouse model of neurodegenerative alpha-synucleinopathies. Furthermore, in vitro ubiquitination of alpha-synuclein fibrils recapitulated the pattern of alpha-synuclein ubiquitination observed in human disease and the A53T alpha-synuclein mouse model. These results suggest that ubiquitination of alpha-synuclein is not required for inclusion formation and follows the fibrillization of alpha-synuclein.