Sam Altman‘s biggest bet doesn’t bear the name OpenAI. It bears the name Helion Energy, a nuclear fusion startup headquartered in Everett, Washington, where the OpenAI CEO has built a personal stake worth approximately $1.7 billion. Forbes ranks it among his top investments, and the numbers speak for themselves: Helion was valued at $15.5 billion in June 2026, following a $465 million Series G round led by Thrive Capital.
Altman first got involved with Helion in 2015, when he was president of Y Combinator and had just persuaded the startup to go through the accelerator. He never stopped investing. In 2021, he led the $500 million Series E round — the single largest check he has ever written as a personal investor. In January 2025, he participated in a further $425 million round alongside SoftBank and Lightspeed Venture Partners, pushing the company’s total fundraising past $1.5 billion. Helion is now building its first commercial plant in Malaga, also in Washington State.
Why fusion, and why now? The answer is straightforward: the data centers powering artificial intelligence models are consuming electricity at a pace existing energy sources struggle to match. Stargate — the $500 billion AI infrastructure initiative launched by OpenAI and SoftBank — requires at least five gigawatts of power. Helion promises virtually unlimited, carbon-free energy through controlled nuclear fusion, and has already signed a supply agreement with Microsoft for 50 megawatts by 2028. It is the world’s first commercial contract for fusion-generated electricity.
Helion’s seventh prototype, known as Polaris, is up and running at the Everett facility. If testing proceeds as planned, the company says it will become the first machine in the world to generate electricity from fusion. CEO David Kirtley has explained that the newly raised funds will be used primarily to bring in-house the production of capacitors, semiconductors, and magnets — components that currently require years of lead time from external suppliers. Manufacturing them internally is designed to speed up reactor construction timelines.
Altman sits on Helion’s board as chairman, but recused himself from negotiations between the startup and OpenAI — a company in which he holds no equity — to avoid any conflict of interest. Reports indicate that OpenAI is considering a deal that could secure it 12.5% of Helion’s output: five gigawatts by 2030 and fifty gigawatts by 2035. These are extraordinary figures for a technology that has yet to demonstrate net energy gain at commercial scale, but the path forward looks considerably more credible today than it did a decade ago. A valuation that tripled in a matter of months — from $5.4 billion to $15.5 billion — suggests the market is starting to believe it.
