Michael Bloomberg will not run for president
He had recently considered running as an independent in November's election.
Mr Bloomberg said he was concerned about the unconventional and insurgent campaigns of Republican Donald Trump and Democrat Bernie Sanders.
The owner of the Bloomberg news organisation said his candidacy would risk letting in Mr Trump or his fellow Republican Ted Cruz.
He set out his reasoning in an article on his website on Monday entitled: "The risk I will not take".
Mr Bloomberg said that his run would split the vote. If there was no clear winner in the popular vote, it would create a situation where Congress would choose the president, he said.
"As the race stands now, with Republicans in charge of both Houses, there is a good chance that my candidacy could lead to the election of Donald Trump or Senator Ted Cruz," Mr Bloomberg wrote.
"That is not a risk I can take in good conscience."
In the end media tycoon Michael Bloomberg had to acknowledge what seemed obvious to many political observers. If he launched an independent presidential bid, he would be likely to hand the White House to someone like Donald Trump or Texas Senator Ted Cruz.
The New Yorker was overly optimistic about his ability to win enough states to throw a deadlocked election to the House of Representatives, but even if he did, outperforming almost every other third-party candidate in modern American history, the outcome would have been the same - a Republican president. Given the two leading candidates' extreme positions on immigration, he wrote, that was just too great a risk for him to take.
Mr Bloomberg's decision can also be read as a tacit acceptance that Hillary Clinton is going to be the Democratic presidential pick. News that he was considering a bid first surfaced as the former secretary of state stumbled in early nominating contests. Now, however, she seems to be pulling away from Bernie Sanders, whose democratic socialism runs counter to Mr Bloomberg's big-business centrism.