In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons (i.e. a thermal system) or fast neutrons. Fissile materials can be used to fuel thermal reactors, with a neutron moderator; fast-neutron reactors, with no moderators; and nuclear explosives.

According to the fissile rule, heavy isotopes with 90 ≤ Z ≤ 100 and 2 × Z – N = 43 ± 2, with few exceptions, are fissile (where N = number of neutrons and Z = number of protons).

"Fissile" is distinct from "fissionable." A nuclide capable of undergoing fission after capturing a neutron is referred to as "fissionable." A fissionable nuclide that can be induced to fission with low energy thermal neutrons is referred to as "fissile." Although the terms were formerly synonymous, fissionable materials include also those (such as uranium-238) that can be fissioned only with high-energy neutrons. As a result, fissile materials (such as uranium-235) are a subset of fissionable materials.