Justin Cohen

Justin Cohen
Occupation	Entrepreneur, engineer
Known for	Co-founder of Maritime Fusion
Education	North Carolina State University (nuclear engineering), Columbia University (plasma physics)

Justin Cohen is an American entrepreneur and engineer who is the co-founder of Maritime Fusion, a company developing compact tokamak fusion reactors designed to power large ships and off-grid energy markets. Maritime Fusion was part of the Y Combinator Winter 2025 batch.

Career

Cohen studied nuclear engineering as an undergraduate at North Carolina State University and subsequently completed graduate work in plasma physics at Columbia University. He has stated that his interest in fusion energy began at age ten after watching the film Iron Man, which depicted a miniature fusion reactor as a power source.

Before founding Maritime Fusion, Cohen worked at SpaceX, where he focused on radiation effects and power electronics. He then spent approximately four years at Tesla, working on battery design, electromagnetic design, motors, chargers, and converters.

Cohen founded Maritime Fusion after concluding that existing fusion companies, while technically capable, lacked a clear path to early market adoption. He observed that first-of-a-kind fusion reactors would likely be too costly and low in capacity to compete on the electrical grid, but could be economically viable in the maritime sector, where power requirements are significantly lower and alternative fuel costs are high. Cohen has noted that this approach mirrors the historical development of nuclear fission technology, which was first deployed in naval vessels such as the USS Nautilus before transitioning to civilian power generation.

Maritime Fusion is building tokamak-based reactors that use high-temperature superconducting (HTS) magnets. The company is also commercializing HTS cable technology for power distribution as a nearer-term revenue path while advancing the physics basis of its reactor design. The company targets both commercial shipping—where there is growing demand for decarbonized propulsion for large vessels—and defense applications. Because fusion reactors do not rely on highly radioactive fuels or produce long-lived radioactive waste, Maritime Fusion contends that its technology faces fewer regulatory challenges than conventional nuclear fission systems.

The Maritime Fusion team includes members with backgrounds at SpaceX, Tesla, Columbia University, and the University of Pennsylvania, with combined experience in plasma physics, nuclear engineering, and electrical engineering. The company is based in San Francisco, California.