硝鎓离子是一种十分特别的离子，化学式为NO₂,虽然全由非金属元素构成，却有金属特性，是鎓离子的一种。

由一个二氧化氮分子被质子化所形成。氮-氧间为双键。 实验室常用制备方式:使硫酸及硝酸反应，产生2个硫酸氢根，一个硝鎓离子及一个水合氢离子，化学式:

2 H₂SO₄ + HNO₃ → 2 HSO₄+ NO₂ + H₃O

The nitronium ion, NO+
2, is a cation. It is an onium ion because of its tetravalent nitrogen atom and +1 charge, similar in that regard to ammonium. It is created by the removal of an electron from the paramagnetic nitrogen dioxide molecule, or the protonation of nitric acid (with removal of H₂O).