三原子氢（H₃）是一种由三个氢原子构成的不稳定分子。

这种中性的分子可以在低压放电管中制备：

它很容易通过下列方式分解：

这种分子只能以激发态存在。这是一些激发态的例子： 2sA₁' 3sA₁' 2pA₂" 3dE' 3DE" 3dA₁' 3pE' 3pA₂"。2pA₂"激发态的寿命为700纳秒。如果分子失去能量并回到低能级，它将迅速自动分解能量最低的介稳态2sA₁'能量为-3.777 eV，比H₃和e状态低，但是只能存在大约1皮秒。

最稳定的状态可能是三氢阳离子获得一个离域电子。

Triatomic hydrogen or H₃ is an unstable triatomic molecule containing only hydrogen. Since this molecule contains only three atoms of hydrogen it is the simplest triatomic molecule and it is relatively simple to numerically solve the quantum mechanics description of the particles. Being unstable the molecule breaks up in under a millionth of a second. Its fleeting lifetime makes it rare, but it is quite commonly formed and destroyed in the universe thanks to the commonness of the trihydrogen cation. The infrared spectrum of H₃ due to vibration and rotation is very similar to that of the ion, H₃. In the early universe this ability to emit infrared light allowed the primordial hydrogen and helium gas to cool down so as to form stars.