硝酸（分子式：HNO₃）是一种强酸，其水溶液俗称硝镪水或镪水或氮酸。纯硝酸为无色液体，沸点83℃，在-42℃时凝结为无色晶体，与水混溶，有强氧化性和腐蚀性。其不同浓度水溶液性质有别，市售浓硝酸为恒沸溶液，溶质质量分数为69.2%，一大气压下沸点为121.6℃，密度为1.42g·cm，约16mol·L，溶质质量分数足够大（市售浓度最高为98%以上）的，称为发烟硝酸，硝酸是一种重要的化工原料。

硝酸的酸酐是五氧化二氮（N₂O₅）。

Nitric acid (HNO₃), also known as aqua fortis and spirit of niter, is a highly corrosive mineral acid.

The pure compound is colorless, but older samples tend to acquire a yellow cast due to decomposition into oxides of nitrogen and water. Most commercially available nitric acid has a concentration of 68% in water. When the solution contains more than 86% HNO₃, it is referred to as fuming nitric acid. Depending on the amount of nitrogen dioxide present, fuming nitric acid is further characterized as white fuming nitric acid or red fuming nitric acid, at concentrations above 95%.