The Farnsworth–Hirsch fusor, or simply fusor, is an apparatus designed by Philo T. Farnsworth to create nuclear fusion. It has also been developed in various incarnations by researchers including Elmore, Tuck, and Watson, and more recently by George H. Miley and Robert W. Bussard. Unlike most controlled fusion systems, which slowly heat a magnetically confined plasma, the fusor injects "high temperature" ions directly into a reaction chamber, thereby avoiding a considerable amount of complexity. The approach is known as inertial electrostatic confinement (IEC).

Hopes at the time were high that it could be quickly developed into a practical source of fusion power. However, as with other fusion experiments, development into a generator has proven to be difficult. Nevertheless, the fusor has since become a practical source of free neutrons, and it is produced commercially for this purpose. Fusors have been assembled in low-power forms by hobbyists.

The fusor was originally conceived by Philo T. Farnsworth, better known for his pioneering work in television. In the early 1930s he investigated a number of vacuum tube designs for use in television, and found one that led to an interesting effect. In this design, which he called the multipactor, electrons moving from one electrode to another were stopped in mid-flight with the proper application of a high-frequency magnetic field. The charge would then accumulate in the center of the tube, leading to high amplification. Unfortunately it also led to high erosion on the electrodes when the electrons eventually hit them, and today the multipactor effect is generally considered a problem to be avoided.