Billionaire Mark Cuban told CNBC on Monday that the stock market's rally from its late-March coronavirus-driven low reminds him of the 1990s dot-com bubble.
"In some respects it's different because of the Fed and the liquidity they've introduced and the inflation for financial assets that comes with that. But on a bigger picture, it's so similar," Cuban said on "Squawk Box."
Cuban, who made billions of dollars during the dot-com boom, pointed to the newfound interest in the stock market from people who weren't interested before, such as his teenage niece.
"I had my 18-year-old niece asking me what stocks she should invest in because her friends are making 30% per day and other people just randomly asking me that never look at stocks at all what stocks they should invest in," said Cuban, who sold Broadcast.com to Yahoo in April 1999 for $5.7 billion.
"Everybody is a genius in a bull market," the "Shark Tank" investor warned. "Everybody is making money right now because you've got the Fed put and that brings people in who otherwise wouldn't participate."
Cuban said he told his niece that the only way to keep the money you make in the stock market is by "cashing out." He said, "Don't get greedy."
As of Friday's close, the tech-heavy Nasdaq had risen nearly 60% since its intraday low on March 23 — the same day the Federal Reserve announced a batch of unprecedented programs to support financial markets, which were cratering due to the coronavirus crisis.
Cuban, who questioned the market's valuation in May, told CNBC on Monday that the internet bubble lasted for multiple years. The Nasdaq rose more than 500% from 1995 until the bubble burst in March 2000.
"It wasn't like, 'Oh, we're in a bubble then all of a sudden the bubble is over months later,'" Cuban said. "It's difficult to have patience sometimes and recognize that there is still a lot money that can come in and chase that performance."
— Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank," which features Mark Cuban as a panelist.