In this manuscript, we provide experimental measurement of the anharmonicity of optical phonons in phenethylammonium lead iodide ((PEA) 2 PbI 4 , Fig. ), a prototypical Ruddlesden-Popper metal halide, by analysing the temperature dependence of the dynamics of impulsively generated coherent phonons. We clearly identify the co-existence of a dominant anharmonic mode with energy of 4.40 meV and various harmonic phonons, the most important one with energy 2.61 meV. We attribute the anharmonic behaviour of the 4.40-meV mode to its decay into two lower-energy acoustic phonons via a three-phonon interaction mechanism, as has been reported in other polar semiconductors . This mechanism has a direct influence on the exciton dephasing rates, especially at higher temperatures. We compare the strength of the anharmonicity of the dominant phonon with that in other semiconductors, and find that it approaches values found in the most highly anharmonic lattices such as SrTiO 3 .