21厘米线，又被称为氢线，21厘米辐射（hydrogen line, 21 centimeter line or HI line）是指由中性氢原子因为能阶变化而产生的电磁波谱线。频率是1420.40575177 MHz，相当于在太空中波长 21.10611405413 公分。在电磁波谱上的位置是微波。 这个波长的辐射经常在射电天文学上被应用，尤其无线电波可以穿过对可见光是不透明的星际云等巨大星际介质区域。

21公分波来自于1s基态氢原子的两个超精细结构之间。两个超精细结构能阶的能量不同，而量子的频率则是由普朗克关系式决定。

The hydrogen line, 21 centimetre line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.405751786 MHz, which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space. This wavelength falls within the microwave radio region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light.