电离能（Ionization energy），或称游离能、电离焓，常简记为E_I，指的是将一个电子自一个孤立的原子、离子或分子移至无限远处所需的能量。更广义的用法，第一电离能定义为气态原子失去一个电子成为一价气态正离子所需的最低能量，记作I₁；气态一价正离子失去一个电子成为气态二价正离子所需的能量称为第二电离能，记作I₂。依此类推。

电离能的数值和原子的有效核电荷密切相关，也和原子大小、轨道中电子间的推斥作用等因素有关。

电离能是了解原子性质的重要数据。

The ionization energy (IE) is qualitatively defined as the amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated gaseous atom to form a cation. It is quantitatively expressed in symbols as:

where X is any atom or molecule capable of being ionized, X is that atom or molecule with an electron removed, and e is the removed electron. This is an endothermic process.