过热蒸汽，或称过热水蒸气，是温度高于水的沸点的蒸汽。饱和蒸汽在常压下加热时，温度持续升高，而产生过热蒸汽。饱和蒸汽表面存在的较高温度的蒸汽，因其温度已超过了饱和温度，而被称做过热蒸汽。

在热平衡状态下，过热蒸汽与液态水是不能共存的，持续加热仅能促成更多液态水的蒸发，转为饱和蒸汽。然而，在动态条件下的某些过热温度点上，是普遍可行的。过热蒸汽并不适用于杀菌。

Superheated steam is a steam at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured.

The steam can therefore cool (lose internal energy) by some amount, resulting in a lowering of its temperature without changing state (i.e., condensing) from a gas, to a mixture of saturated vapor and liquid. If saturated steam (a mixture of both gas and saturated vapor) is heated at constant pressure, its temperature will also remain constant as the vapor quality (think dryness, or percent saturated vapor) increases towards 100%, and becomes dry (i.e., no saturated liquid) saturated steam. Continued heat input will then "super" heat the dry saturated steam. This will occur if saturated steam contacts a surface with a higher temperature.