He is planning to submit his name for the Democratic presidential Primary in Alabama on Friday, the deadline to do so.
He has not decided whether to run for president but is back to actively considering it and is taking this step to keep his options open.
The news was first reported by The New York Times and confirmed by ABC News.
"Mike believes that Donald Trump represents an unprecedented threat to our nation," Howard Wolfson, an adviser to Bloomberg, said in a statement Thursday evening. "In 2016, he spoke out at the Democratic Convention, warning against a Trump presidency. In 2018, he spent more than $100 million to help elect Democrats to ensure that Congress began to hold the president accountable. And this year he helped Democrats win control of both houses of the Virginia legislature. We now need to finish the job and ensure that Trump is defeated -- but Mike is increasingly concerned that the current field of candidates is not well positioned to do that."
"Based on his record of accomplishment, leadership and his ability to bring people together to drive change," Wolfson added, "Mike would be able to take the fight to Trump and win.”
Last year, Bloomberg announced on Instagram that he decided to re-register as a Democrat, a move many interpreted as a strong indication he had interest in a presidential bid.
"At key points in U.S. history, one of the two parties has served as a bulwark against those who threaten our Constitution," the billionaire wrote in October 2018. "Two years ago at the Democratic Convention, I warned of those threats."
"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," he added.
Two months later, Bloomberg reportedly held meetings with top Iowa Democrats during a multi-city tour through the first state that has a say in a presidential race.
Howard Schultz, the billionaire former Starbucks CEO, had considered a 2020 run against Trump, also a billionaire.
"More billionaires seeking more political power," said Faiz Shakir, Bernie Sanders' campaign manager, "surely isn't the change America needs."
Bloomberg has outspokenly opposed President Donald Trump and his policies since the 2016 campaign, calling him a "dangerous demagogue" in his endorsement of Hillary Clinton at the Democratic National Convention.
"Trump says he wants to run the nation like he's running his business? God help us. I'm a New Yorker, and I know a con when I see one," Bloomberg said previously.
Trump quickly fired back, referring to Bloomberg as "little" at a July 2016 rally in Iowa. He repeated the insult on Twitter and said Bloomberg's last term as New York City mayor was a "disaster."
After learning of Bloomberg's presidential ambitions, the president told the New York Post he would "love" to run against him in 2020.
"I'd love to run against Little Michael," Trump told the Post. "He's been fighting me hard. He spent $100 million against me -- that didn't work."
Bloomberg left the Democratic party in 2001 to run for New York City mayor as a Republican. In 2007, he left the Republican party to register as an Independent. In September, Bloomberg told the New York Times, he believes "only a major-party nominee can win the White House."
Bloomberg was born Feb. 14, 1942, in Boston and hails from Medford, Massachusetts. He was an Eagle Scout who attended public schools.
The former mayor later attended Johns Hopkins University as an undergraduate. Heavily involved in student government, he was elected president of his fraternity, president of the Inter-Fraternity Council and class president.
He paid for his tuition by working as a parking lot attendant and taking out loans. After graduating in 1964 from Johns Hopkins with a B.A. in electrical engineering, he earned an M.B.A. from Harvard Business School.
Bloomberg, now a philanthropist after returning to Bloomberg LP following his three terms as mayor, in 1966 went to work for the Wall Street firm, Salomon Brothers. He worked there 15 years, making partner in 1972, general partner in 1976 as the head of Equity Trading and Sales and running Information Systems in 1979. Bloomberg was let go from the firm in 1981 when it was acquired.
Bloomberg used his severance from Salmon Brothers to found Bloomberg LP, the parent company of Bloomberg News, which includes TV and radio operations and employs nearly 20,000 people.
In 2001, he was elected New York City's 108th Mayor, a few weeks after the Sept. 11 terror attacks.
ABC News' Averi Harper, Rick Klein and Meg Cunningham contributed to this report.