The move is being seen as a possible step toward running for president in 2020.
Bloomberg, 76, has recently said he is actively considering running for president as a Democrat.
Changing his New York State voting registration is his most overt move to date toward a possible White House campaign.
A Democrat before seeking elective office, Bloomberg switched his party registration in 2001 to run for mayor as a Republican, before abandoning political affiliations in 2007. He won his third term in 2009 as an independent.
Mayor Bloomberg made the announcement on Instagram Wednesday morning:
He also shared the announcement on his Facebook and Twitter accounts.
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