Fusion ignition is the point at which a nuclear fusion reaction becomes self-sustaining. This occurs when the energy being given off by the reaction heats the fuel mass more rapidly than it cools. In other words, fusion ignition is the point at which the increasing self-heating of the nuclear fusion removes the need for external heating. This is quantified by the Lawson criterion. Ignition can also be defined by the fusion energy gain factor.

In the laboratory, fusion ignition defined by the Lawson criterion was first achieved in August 2021, and ignition defined by the energy gain factor was achieved in December 2022, both by the U.S. National Ignition Facility.

Ignition should not be confused with breakeven, a similar concept that compares the total energy being given off to the energy being used to heat the fuel. The key difference is that breakeven ignores losses to the surroundings, which do not contribute to heating the fuel, and thus are not able to make the reaction self-sustaining. Breakeven is an important goal in the fusion energy field, but ignition is required for a practical energy producing design.

In nature, stars reach ignition at temperatures similar to that of the Sun, around 15 million kelvins (27 million degrees F). Stars are so large that the fusion products will almost always interact with the plasma before their energy can be lost to the environment at the outside of the star. In comparison, man-made reactors are far less dense and much smaller, allowing the fusion products to easily escape the fuel. To offset this, much higher rates of fusion are required, and thus much higher temperatures; most man-made fusion reactors are designed to work at temperatures over 100 million kelvins (180 million degrees F).

Fusion ignition was first achieved by humans in the cores of detonating thermonuclear weapons. A thermonuclear weapon uses a conventional fission (U-235 or Pu-239/241) "sparkplug" to generate high pressures and compress a rod of fusion fuel (usually lithium deuteride). The fuel reaches high enough pressures and densities to ignite, releasing large amounts of energy and neutrons in the process.

The National Ignition Facility at Lawrence Livermore National Laboratory performs laser-driven inertial confinement fusion experiments that achieve fusion ignition. This is similar to a thermonuclear weapon, but the National Ignition Facility uses a 1.8 MJ laser system instead of a fission weapon to compress the fuel, and uses a much smaller amount of fuel (a mixture of deuterium and tritium, which are both isotopes of hydrogen). In January 2012, National Ignition Facility Director Mike Dunne predicted in a Photonics West 2012 plenary talk that ignition would be achieved at NIF by October 2012. By 2022 the NIF had achieved ignition.^{[citation needed]}

Based on the tokamak reactor design, the ITER is intended to sustain fusion mostly by internal fusion heating and yield in its plasma a ten-fold return on power. Construction is expected to be completed in 2025.^{[citation needed]}

Experts believe that achieving fusion ignition is the first step towards electricity generation using fusion power.