西门子 （英语：Siemens） ，是物理电路学及国际单位制中，电导、电纳和导纳，三种导抗的单位。

西门子的符号为S，中文简写时为「西」，英文全写时应为小写的siemens。名字出处是为了纪念德国电学家、发明家和工业家维尔纳·冯·西门子。

由于它是电阻、电抗和阻抗的单位──欧姆(Ω)的倒数，故此又与：

同等，但以上两者皆已不被普遍采用。

其他单位转换及量纲式：
1 S

The siemens (SI unit symbol: S) is the unit of electric conductance, electric susceptance and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is equal to the reciprocal of one ohm, and is also referred to as the mho. The 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.