A vibrational transition is an abrupt change in vibrational energy in quantum physics. Like all other properties of a quantum particle, vibrational energy is quantized, meaning it can only equal certain discrete values, which correspond to different energy states. When a particle loses a unit of vibrational energy, known as a phonon, it is said to have transitioned to a lower vibrational energy state. Likewise, when a particle gains vibrational energy, a positive vibrational transition is said to have occurred.

Vibrational transitions are important in physics due to the unique spectral lines that result. Because there is a net gain or loss of energy during a transition, electromagnetic radiation of a particular frequency must be absorbed or emitted. This forms spectral lines at that frequency which can be detected with a spectrometer, as in Mass Spectrometry or Raman spectroscopy.

Vibrational transitions are strongly related to photon-phonon coupling, a phenomenon that allows some lasers to be tunable, like the Ti-sapphire laser.

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