someone once gave him some bitcoin for his birthday, but he sold it a few years afterward.

Microsoft co-founder Bill Gates said Monday he would bet against bitcoin if he could.

"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.