CNBC's Jim Cramer said Monday he thinks cryptocurrency markets may put in a near-term top thanks to the impending launch of the first bitcoin futures exchange-traded fund in the U.S.

As a result, the "Mad Money" host said he decided to exercise some discipline and trim his holdings of ether, which runs on the Ethereum blockchain and is the world's second-largest cryptocurrency by market value.

"Cryptocurrencies, they've become unstoppable, and even as someone who likes them, I think they've become the definition of rank speculation," Cramer said. "You could argue they're roaring because people want insurance against inflation, but I think they're roaring because a crypto ETF is launching tomorrow and people want in ahead of time."

"If I'm right, then tomorrow very well could be the peak for crypto, and that's why I sold off one-eighth of my Ethereum position today," he continued. "Obviously, I would sell it all if I had any confidence that it was the top, but I don't. It's been too big a win for me."

Cramer indicated last week that he may sell a chunk of his ether holdings if a bitcoin-related ETF were to start trading, but Monday's comments offer insight into the market commentator's specific trading decision.

Ether mounted a strong rally in recent months, after struggling from mid-May when it traded above $4,000 per token, until late July when it dipped below $1,800. It traded around $3,730 Monday evening, according to Coin Metrics. Ether is up roughly 400% year to date.

Bitcoin, the world's largest cryptocurrency by market value, was higher by nearly 2% on Monday to around $61,700 per token. It's been on a tear in recent weeks, after trading below $42,000 at times in late September.