"It's just not a real thing, eventually it will be closed," Dimon said Tuesday at the Delivering Alpha conference presented by CNBC and Institutional Investor.
Dimon joked that even his daughter bought some bitcoin, looking to cash in on a trend that has seen it soar more than 300 percent this year.
"I'm not saying 'go short bitcoin and sell $100,000 of bitcoin before it goes down," he said. "This is not advice of what to do. My daughter bought bitcoin, it went up and now she thinks she's a genius."
In an appearance at a separate conference earlier in the day, Dimon said bitcoin mania is reminiscent of the tulip bulb craze in the 17th century.
"It's worse than tulip bulbs. It won't end well. Someone is going to get killed," Dimon said at a banking industry conference organized by Barclays. "Currencies have legal support. It will blow up."
Dimon also said he'd "fire in a second" any JPMorgan trader who was trading bitcoin, noting two reasons: "It's against our rules and they are stupid."
Bitcoin fell to its session lows after Dimon's comments. As of 3:01 p.m. in New York, bitcoin traded at $4,106.23, down 2 percent.
Dimon's criticism comes at a time when some of the most well-known figures on Wall Street are starting to embrace the cryptocurrency. Fundstrat's Tom Lee said he sees bitcoin surging to $6,000 next year and value investor Bill Miller reportedly owns bitcoin.