Stocks in the US surged in the aftermath of Trump's election, as investors anticipated policies that would shake up the economy.
"Markets see Trump dismantling regulations and reducing taxes, and that has been the dream," Soros, chairman of Soros Fund Management, told Bloomberg.
However, Soros warned that "the dream" will not last, adding: "It's impossible to predict exactly how Trump is going to act."
"Uncertainty is at a peak, and actually uncertainty is the enemy of long-term investment," he said. "I don’t think the markets are going to do very well. Right now they’re still celebrating. But when reality comes, it will prevail."
And in general, Soros is not optimistic about how Trump will pan out as US President.
"I personally am convinced that he is going to fail. Not because of people like me who would like him to fail," he said.
"But because his ideas that guide him are inherently self-contradictory and the contradictions are actually already embodied by his advisers ... and his cabinet."
Soros has form with dire warnings - ahead of the UK's EU referendum in June, he predicted the value of sterling would fall dramatically in Britons voted in favour of Brexit.
Trump's inauguration ceremony takes place from 4.30pm UK time this afternoon - all the details are here.