The billionaire, who served as Mayor of New York as both a Republican and an independent, has not ruled out running for president in the 2020 US election.
Billionaire businessman and former New York mayor Michael Bloomberg, who has not yet ruled out a presidential run in 2020, said Wednesday he has registered as a Democrat, switching camps again.
"At key points in US history, one of the two parties has served as a bulwark against those who threaten our Constitution," said Bloomberg, who in the past has identified as a Republican and an independent.
"Two years ago at the Democratic Convention, I warned of those threats," the 76-year-old Bloomberg said on Instagram.
"Today, I have re-registered as a Democrat – I had been a member for most of my life – because we need Democrats to provide the checks and balance our nation so badly needs."
In the United States, when voters register with a particular party, it means they plan to vote most often for that party's candidates. Voters are not required to be registered with any political movement.
Bloomberg, the founder of the financial data and media company that bears his name, has said he will decide in the wake of the November 6 midterm elections whether he will make a run for the White House.
The billionaire - who served as mayor of New York from 2002 to 2013, both as a Republican and an independent - had already said that if he were to launch a presidential campaign, it would be as a Democrat.
Bloomberg, who in the past has donated to both Democratic and Republican candidates, has said he will spend $80 million on Democrats alone ahead of November's key polls.