热发射（英语：thermionic emission）是一种通过热激发发射载流子的方式。这个现象发生的原因是，提供给载流子的热能使它们能够克服束缚势能（在金属材料中，这束缚势能也被称为功函数或逸出功）。通过热发射产生的载流子可能是电子或者离子。发射载流子之后原始区域会产生一个于被发射载流子总和大小相同、极性相反的载流子。不过，如果发射极连接在电池上，则物体上产生的电荷会立即被电池提供的载流子中和掉，最终发射极会达到电平衡，重新回到之前的状态。产生电子的热发射被称为热电子发射。

Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the work function of the material. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as "thermions". After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission.