里德伯原子是指具有高激发态电子（主量子数n很大）的原子。里德伯原子中只有一个电子处于很高的激发态，离原子实（原子核和其余的电子）很远，原子实对这个电子的库仑作用可视为一个点电荷的库仑作用，因此可以将里德伯原子看作类氢原子，将多体问题转化为单电子问题，这样就大大简化了计算。

1885年巴耳末提出氢原子光谱的巴耳末公式之后，就有人观测到了n=13的氢原子谱线。1893年美国天文学家皮克林在天文观测中观测到了n=31的谱线。1906年又有人观测到了n=51的钠的里德伯原子。目前人们在实验室中已经制备出n≈105的原子，射电天文已经观测到了n≈630的里德伯原子。

The Rydberg states of an atom or molecule are electronically excited states with energies that follow the Rydberg formula as they converge on an ionic state with an ionization energy. Although the Rydberg formula was developed to describe atomic energy levels, it has been used to describe many other systems that have electronic structure roughly similar to atomic hydrogen. In general, at sufficiently high principal quantum numbers, an excited electron - ionic core system will have the general character of a hydrogenic system and the energy levels will follow the Rydberg formula. Rydberg states have energies converging on the energy of the ion. The ionization energy threshold is the energy required to completely liberate an electron from the ionic core of an atom or molecule. In practice, a Rydberg wave packet is created by a laser pulse on a hydrogenic atom and thus populates a superposition of Rydberg states. Modern investigations using pump-probe experiments show molecular pathways – e.g. dissociation of (NO)₂ – via these special states.