Speaking to a packed audience at his annual dinner at the World Economic Forum in Davos, Mr Soros said he was confident Mr Trump's powers would be limited by Congress.
"He won't be able to get away [with being a dictator]," he added.
Ultimately, Mr Soros said he expected Mr Trump to fail.
"The ideas that guide him are fundamentally contradictory. He has three chiefs of staff instead of one and his cabinet are very fragmented and fighting with each other," he said.
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Mr Soros has been unrelentingly critical of Mr Trump since his shock election victory, and the pair have a history of friction.
During the 2016 campaign, Mr Soros donated around $10m to Trump rival Hillary Clinton.
The hedge fund manager, who initially became famous for having made $1bn by betting on the devaluation of the pound in 1992, is reported to have lost close to $1bn after the stock market rallied following Mr Trump's win.
Mr Soros attributed the rise in the markets to Mr Trump's pledge to cut regulation and taxes, but said when Mr Trump takes office - in under 24 hours - "reality will prevail".
He also said that it's impossible to predict exactly how Mr Trump will act because he hadn't expected to win, and had only started thinking seriously about what he would do after he was elected.
He also warned that Mr Trump would also divide the US further.
"Anyone who disagrees with him is not part of the people," he says.
Asked what advice, he would give to businesses preparing for the impact of the new presidency, he said, "I'd keep as far away from it as I can."
Mr Trump's team was approached for comment by the BBC.
"An imposter, a [political] conman and a would-be dictator," is the verdict of billionaire investor George Soros on Donald Trump.