Why Union Street bears its name is uncertain.
Forgotten New York, a local history site, has given two explanations. A 2011 article says Brooklyn’s Union Street, like Union streets and avenues elsewhere in the city, uses ‘the Union’, a once-common name for the United States. Later articles by the same author say it is named for the Union Stores, a 19th-century East River dockside warehouse for sugar and molasses.