Ken Griffin, CEO of investment firm Citadel and a prominent crypto detractor, seems to have softened his stance on digital currencies.

Griffin previously dismissed cryptocurrencies as "a jihadist call" against the dollar, but this week said that "crypto has been one of the greatest stories in finance over the course over the last 15 years."

"The crypto market today has a market capitalization of about $2 trillion in round numbers, which tells you that I haven't been right on this call," he said in an interview with Bloomberg. In crypto, market capitalization is the value of all the coins that have been minted.

Griffin — the No. 40 richest person in the world with a net worth of about $30.4 billion, according to Bloomberg — said that while he is still not completely sold on crypto, Citadel cannot ignore the space any longer.

"I still have my skepticism, but there are hundreds and millions of people in this world today who disagree with that," he said. "To the extent that we're trying to help institutions and investors solve their portfolio allocation problems, we have to give serious consideration to being a market maker in crypto."

He added that it is "fair to assume" that Citadel will begin to engage with cryptocurrencies "over the months to come."

It's a major reversal for Griffin, who in 2017 told CNBC that bitcoin has "many of the elements of the tulip bulb mania," referring to the period in the 1630s when the price of tulip bulbs skyrocketed before crashing.

In the same interview, Griffin said he worried that crypto investors "don't really understand what they're participating in" and warned that "bubbles tend to end in tears." Last February, Griffin told CNBC that he doesn't "spend much time thinking about cryptocurrencies."

In November, Griffin made headlines in the crypto space when he paid $43.2 million for a first-edition copy of the U.S. Constitution. The copy had been a target of ConstitutionDAO, a decentralized autonomous organization that had raised more than $40 million in ether from its collective members, which included others in the cryptocurrency space and institutional investors like cryptocurrency exchange Gemini, to attempt to purchase the document.

"The U.S. Constitution is a sacred document that enshrines the rights of every American and all those who aspire to be," Griffin said after buying the copy. "That is why I intend to ensure that this copy of our Constitution will be available for all Americans and visitors to view and appreciate in our museums and other public spaces."

Anyone interested in investing in crypto should keep in mind that experts recommend doing your research and understanding the risks. Cryptocurrencies are notoriously volatile, and you should only invest money that you can afford to lose.

Sign up now: Get smarter about your money and career with our weekly newsletter

Don't miss: Got a suspicious phone call? Here's how to avoid falling for a tax scam