Commonwealth Fusion Systems LLC said it has raised more than $1.8 billion in the largest private investment for nuclear fusion yet as startups race to be the first to generate carbon-free energy like the sun.

Big-name investors backing the latest funding round for the Massachusetts-based company include Microsoft Corp. co-founder Bill Gates and George Soros via his Soros Fund Management LLC. Some of Commonwealth Fusion’s competitors, including Helion Energy Inc., have also recently secured huge funding as investors pile into clean energy technologies amid growing concerns about climate change.

Nuclear fusion has long been the holy grail of the energy world. Fusion is the process of generating energy by melding atoms. Current nuclear power plants create energy through nuclear fission, or splitting atoms. Fusion has the potential to create nearly limitless energy using common elements such as hydrogen, and has the added benefit of generating little to no long-lived nuclear waste.

But despite decades of research, no one to date has been able to produce net energy through fusion—or more energy than it takes to create a fusion reaction. Private firms are vying to be the first not only to create net-energy machines, but to commercialize them by delivering electricity to the grid on the scale of a power plant.

“Everything is science fiction until someone does it and then all of a sudden it goes from impossible to inevitable,” said Bob Mumgaard, chief executive of Commonwealth Fusion, which was spun out of the Massachusetts Institute of Technology in 2018.

The recent infusion of cash into fusion startups eclipses the roughly $1.9 billion in total that was previously announced, according to data tracked by the Fusion Industry Association and the U.K. Atomic Energy Authority.

Helion Energy announced in early November that it had raised $500 million, with another $1.7 billion committed that is tied to meeting performance milestones. Canada’s General Fusion this week closed a $130 million fundraising round that was oversubscribed, said Chief Executive Christofer Mowry. New investors included a state pension fund and the hedge fund Segra Capital Management.

“It’s a sign of the industry growing up,” Mr. Mowry said. General Fusion plans to launch a larger fundraising effort next year.

Companies are pursuing different designs for fusion reactors, but most rely on fusion that takes place in plasma, a hot charged gas. In September, Commonwealth Fusion successfully tested the most powerful fusion magnet of its kind on Earth that would hold and compress the plasma.