碳鎓离子（英语：Carbonium ion）是非经典类型的五配位或四配位碳正离子，比如五配位的离子R₅C。

母体化合物甲鎓离子或写作CH₅是甲烷被超强酸质子化的产物。这种离子是存在于星际介质中的活性中间体，实验室里也可以在低温低浓度的气相中制备。这种离子被认为是CH₃碳正离子与一分子的氢的空轨道相互作用，并形成一个三中心二电子键。来自氢分子的两个氢原子能够与来自CH₃的三个氢原子不断交换位置，所以甲鎓离子被认为是流变分子。交换反应的能垒很低，甚至在绝对零度下也能发生。 甲鎓离子的构象信息可通过红外光谱得到。

In chemistry, carbonium ion is any cation that has a pentavalent carbon atom, The name carbonium may also be used for the simplest member of the class, properly called methanium (CH+
5), where the five valences are filled with hydrogen atoms.

The next simplest carbonium ions after methanium have two carbons. Ethanium or protonated acetylene C
2H+
3 and ethenium C
2H+
5 are usually classified in other families. The ethanium ion C
2H+
7 has been studied as an extremely rarefied gas by infrared spectroscopy.