"As an asset class, you're not producing anything and so you shouldn't expect it to go up. It's kind of a pure 'greater fool theory' type of investment," Gates said on CNBC's "Squawk Box."
"I agree I would short it if there was an easy way to do it," he said.
The price of one bitcoin briefly soared $2,000 last year to above $19,000 in mid-December. The cryptocurrency has since lost more than half its value and was trading near $9,300 on Monday morning. Roughly $9.8 billion has been raised through sales of new coins, or ICOs, since 2016, according to financial research firm Autonomous Next.
"Bitcoin and ICOs, I believe completely [they're some] of the crazier, speculative things," Gates said. He added someone once gave him some bitcoin for his birthday, but he sold it a few years afterward.
Cryptocurrencies can in fact be shorted — futures and put options for bitcoin have been made available through some exchanges, for instance — but making bets against virtual currencies is not as easy as it is for more established assets such as corporate shares.
Like many critics of the cryptocurrency price surge, Gates did say the underlying blockchain technology had its merits.
Blockchain eliminates the need for a third-party intermediary, such as a bank, by quickly creating a secure, permanent record of a transaction between two parties. Bitcoin is the first application of blockchain technology, and companies are exploring ways to apply blockchain to supply chain management, trading and other areas.
Gates is on the board of Warren Buffett's Berkshire Hathaway and was speaking from Omaha, Nebraska, where the conglomerate held its annual shareholders meeting over the weekend. Billionaire investor Buffett and his longtime investing partner and vice chairman, Charlie Munger, spoke to the tens of thousands attendees on a wide range of topics, including bashing bitcoin as "rat poison."
With Berkshire's 2018 annual meeting in the books, users can revisit the highlights in CNBC's Warren Buffett Archive, which houses searchable video from 25 full annual meetings, going back to 1994, synchronized to 2,600 pages of transcripts. The Warren Buffett Archive also includes 500 shorter-form videos arranged by topic, CNBC interviews, a Buffett Timeline, and a Berkshire Portfolio Tracker.