The round brings Commonwealth Fusion Systems to $4 billion raised in total.
The company did not identify the new investors, saying only that the round included pension funds, sovereign wealth funds, and industrial partners.
The funding comes as CFS assembles SPARC, its compact tokamak in Devens, Massachusetts.
The company expects SPARC to reach scientific breakeven in 2027.
That means the plasma produces more fusion energy than is used to heat it. It does not mean electricity is reaching the grid.
CFS is also finalizing the design of ARC, its planned first commercial power plant in Virginia.