The plant circadian clock regulates the rhythms of plant metabolism. Many herbivore-induced plant volatiles (HIPVs) fluctuate, diurnally, but the role of the circadian clock in the emission of HIPVs and their ecological consequences remains largely unknown. Here, we show that the timing of herbivore attack can alter the outcome of tri-trophic interactions, and this is mediated by the circadian clock, under both field and glasshouse conditions. Although most HIPV emissions did not have a circadian rhythm, the circadian clock modulated HIPV emissions in a time-dependent manner. HIPVs mediate tri-trophic interactions, and the circadian clock may affect these interactions by modulating HIPV emission in nature.