"I think a hard Brexit will be a disaster for Great Britain," Dimon said at the Economic Club of New York. "We don't think it will happen because it's bad for Europe too."
A hard Brexit refers to the U.K. leaving the EU without access to the bloc's single market and customs union.
Dimon's comments came a day after the British Parliament shot down a proposed Brexit deal by Prime Minister Theresa May's government. That proposed deal had been previously approved by the EU. The deal's defeat throws the exit process into chaos as the U.K. now has until Jan. 21 to push through a new agreement. It also led Labour Party leader Jeremy Corbyn to call for a vote of no confidence in the government. May won the vote later on Wednesday.
"The Brits were dealt a bad hand and they played it badly," Dimon said. "I'm hoping they have a withdrawal agreement that gives them two to four years to negotiate a real deal."
"We spent hundreds of millions of dollars to be prepared but a lot of people aren't," Dimon added about his bank's plans.
A majority of U.K. voters in June 2016 backed leaving the EU. That decision sent ripples through global markets. The pound, for example, has lost more than 14 percent of its value against the dollar since then.