Billionaire Stephen Mandel Just Loaded Up on This Incredible Artificial Intelligence (AI) Stock Up Nearly 200% in 2026

The latest round of Form 13Fs is out, which means investors can track what billionaire investors are doing. One hedge fund manager I follow is Stephen Mandel, who runs Lone Pine Capital. Mandel has a phenomenal track record, and isn't just a tech investor; he's diversified across all sectors.

What makes his latest move in Q2 so significant is that he added a stock to his portfolio that wasn't there during Q1 and made it his largest overall holding. That's a major vote of confidence, and clues investors in that they should investigate further.

What is this stock? It's Nebius (NASDAQ: NBIS). Here's why he might have bought it.

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Nebius is one of the fastest-growing stocks on the market

Nebius has had an incredible 2026 and is up nearly 200% this year. It had a major dip at the end of July, but that doesn't matter.

These filings only include holdings as of June 30, so they wouldn't reflect whether Mandel bought the July dip, but if he bought in April, he's still up a lot on his initial investment. Nebius caught fire throughout the summer and emerged as one of the best stock picks of 2026, mainly because of its growth. However, there are plenty of signs that Nebius can continue this for the foreseeable future, making it still a great stock to consider owning.

Nebius operates in the neocloud space and is an AI-focused cloud computing provider. Right now, there is a major shortage of AI computing power, and any company that has access to computing capacity is in a great spot. To sweeten the deal, Nebius is a preferred partner of Nvidia and gains early access to cutting-edge technology. This makes it a top provider to consider, as Nebius will always have the latest and greatest products.

Nebius has signed deals with several of the major AI hyperscalers to rent them computing power, and this is leading to jaw-dropping growth. In Q2, Nebius grew revenue by 454% year over year. That number wasn't boosted by an acquisition or one-time effect; that's real growth coming from rapidly expanding its computing capacity. It's also not a one-off quarter, either. Analysts expect 523% growth for the second half of 2026 and 258% growth for 2027. With AI computing demand only expected to increase, this makes Nebius one of the best options in terms of pure growth in the AI space.

Although Nebius' stock has about tripled in 2026, I still think it's a phenomenal investment to make, and you should consider following billionaire Stephen Mandel's lead and purchasing shares.

Keithen Drury has positions in Nebius Group and Nvidia. The Motley Fool has positions in and recommends Nvidia. The Motley Fool has a disclosure policy.