Cuban, a supporter of Democrat Hillary Clinton and outspoken Trump critic, said the Republican nominee's temperament poses a serious risk to markets. A "stupid and offensive" Trump remark during his term could cause geopolitical discord and hit global markets, the entrepreneur said.
"That is the worst thing possible for the stock market," Cuban told CNBC's "Squawk Alley," adding that the U.S. stock market fell after Friday's news that the FBI was looking into new Clinton-related emails.
Cuban has repeatedly hammered Trump for his business acumen and potential to drag the stock market down. The billionaire developer has touted his business success on the campaign trail as evidence of his ability to be president.
However, he said Trump's bizarre remarks show he lacks fitness for the presidency and voters should not expect that to change.
"The thought that once he was elected or if he's elected that all the sudden now with all this potential power he's just going to keep quiet and stay scripted. There's no chance that happens. ... The levels to which he would be corrupted by that power are just beyond conception right now," Cuban said.
He added that he sees risks in a Clinton presidency in that some of her proposals favored by the far-left wing of the Democratic Party, like backing out of or reforming some trade deals or expanding government spending on college. Cuban said he does not agree with her on all policies but "she understands how government works enough that I can lobby against her."
Trump campaign spokeswoman Hope Hicks responded to Cuban's remarks in a statement, saying that "I don't think anyone affiliated with Hillary Clinton has much credibility when speaking about corruption. Furthermore, unlike Mark Cuban, Mr. Trump has a winning temperament and his message has resonated with Americans across the country."