In 2025, Sam Altman doubled down on ambitious startups tackling power, health and automation. Sean Gallup/Getty Images

Aside from becoming a household name with the breakout success of ChatGPT in 2022, OpenAI CEO Sam Altman has long been one of Silicon Valley’s most influential startup investors. Long before generative A.I. captured global attention, Altman built a reputation as a prolific angel investor whose bets helped shape the modern tech landscape, from early stakes in Stripe, Reddit and Instacart to later, high-conviction investments in nuclear energy and longevity science.

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Altman’s track record as an early-stage investor was shaped during his time at Y Combinator, where he served as president from 2014 to 2019. In that role, he helped oversee investments in hundreds of startups, several of which went on to become major technology companies. After leaving YC, Altman continued to invest personally in both alumni companies and new ventures.

Over the years, Altman has invested in more than 400 companies through a mix of personal angel checks, his own funds like Hydrazine Capital and Apollo Projects, and partnerships with larger venture firms.

Altman’s 2025 investment activity has centered on startups in A.I.-driven security, biotechnology and nuclear fusion, according to Crunchbase data. These bets reflect not just a desire to diversify across cutting-edge industries, but also to champion “moonshot” ventures tackling some of today’s most complex challenges, from intelligence augmentation and clean energy to breakthroughs in human health.

Here are the startups that Altman invested in this year:

Helion Energy

• Product: fusion power
• Investment........
  

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