• Sam Altman said he owns about a third of Helion Energy, a nuclear fusion power startup.
• An exhibit in his trial against Elon Musk gave unusual clarity to his finances.
• Altman is a billionaire, but not from shares in OpenAI.

Sam Altman on Tuesday said his investment in nuclear energy startup Helion, which is deep in talks on a power purchasing agreement with OpenAI, has climbed to over $1.6 billion in value.

That's up from the $375 million he invested in the company in November 2021, leading its Series E round.

While testifying at the trial over OpenAI's for-profit ambitions, Altman was presented with a document he'd signed on April 24. The exhibit gave an unusually clear picture of Altman's financial standing, showing the value of his holdings in a slate of buzzy startups.

Altman confirmed on the witness stand that he had a stake in Helion worth more than $1.6 billion as of the end of 2025. He said his first investment in the nuclear startup was in 2015, and that he's participated in multiple funding rounds since. He estimated that he owns around a third of the company, and said he also owns warrants to purchase more shares.

The exhibit also showed multimillion-dollar stakes in other tech companies. It said Altman owns $632 million of Stripe, the valuable fintech startup, and a $258 million stake in the antiaging research firm Retro Biosciences. It also listed smaller stakes, including $19 million in Trialspark, a pharmaceutical company that's rebranded to the name Formation Bio, and $3 million in Cerebras, an AI chipmaker currently preparing its IPO.

Altman's investments have come into focus at the trial, where Elon Musk is seeking to prove that Altman and other founders have taken to "looting" the charity, which Musk helped start in 2015, by joining forces with Microsoft to make money from ChatGPT.

Former OpenAI board member Shivon Zilis testified that she was on the board when she first heard about a potential OpenAI deal with Helion. She said she was concerned about the pitch because both Altman and Greg Brockman were investors.

It "felt super out of left field," she said. "How is it the case that we want to place a major bet on a speculative technology?"

On Tuesday, Musk attorney Steven Molo grilled Altman about a planned power deal between Helion and OpenAI, and whether Altman was playing both sides of the agreement. Altman said that he spoke with people at OpenAI about the deal, but that he recused himself on both sides: "I didn't sign it, I think OpenAI signed it."

"Do you think recusal means only that you didn't sign it?" Molo said.

"No, I think recusal means the decision to proceed, and the final approval of terms," Altman replied. He stepped down as Helion's board chair in March 2026.

Molo jumped next to Altman's ties to Reddit, where he once briefly served as interim CEO; he also built up a sizable stake directly and in venture funds. After establishing that Altman controlled somewhere in the ballpark of $300 million in Reddit shares around its March 2024 IPO, Molo pointed to a major content licensing deal that Reddit and OpenAI announced a couple of months later.

"You had an obvious conflict in regard to that deal, right?" Molo asked.

"Yes," Altman said. He explained that he went through a recusal process with his company, and Molo's train of questions was cut off by the judge's call for recess.