自发发射（Spontaneous emission），是在没有任何外界作用下，激发态原子或是分子的电子自发地从高能阶向低能阶跃迁，同时发射出一光子。

各原子的自发发射过程完全是随机的，所以自发辐射光是非相干的。

Spontaneous emission is the process by which a quantum system such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy (e.g., the ground state) and emits quanta of energy. Light or luminescence from an atom is a fundamental process that plays an essential role in many phenomena in nature and forms the basis of many applications, such as fluorescent tubes, older television screens (cathode ray tubes), plasma display panels, lasers, and light emitting diodes. Lasers start by spontaneous emission, and then normal continuous operation works by stimulated emission.