In this work, we derive the asymptotic quasi-normal modes of a Banados–Teitelboim–Zanelli (BTZ) black hole using a quantum field theoretic Lagrangian. The BTZ black hole is a very popular system in the context of 2 + 1-dimensional quantum gravity. However, to our knowledge the quasi-normal modes of the BTZ black hole have been studied only in the classical domain. Here we show a way to quantize the quasi-normal modes of the BTZ black hole by mapping it to the Bateman–Feschbach–Tikochinsky oscillator and the Caldirola–Kanai oscillator. We have also discussed a couple of other black hole potentials to which this method can be applied.