Until recently it was believed that photosynthesis, a fundamental process for life on earth, could be fully understood with semiclassical models. However, puzzling quantum phenomena have been observed in several photosynthetic pigment–protein complexes, prompting questions regarding the nature and role of these effects. Recent attention has focused on discrete vibrational modes that are resonant or quasi-resonant with excitonic energy splittings and strongly coupled to these excitonic states. Here we unambiguously identify excited state coherent superpositions in photosynthetic light-harvesting complexes using a new experimental approach. Decoherence on the time scale of the excited state lifetime allows low energy (56 cm^–1) oscillations on the signal intensity to be observed. In conjunction with an appropriate model, these oscillations provide clear and direct experimental evidence that the persistent coherences observed originate from quantum superpositions among vibronic excited states.