Many species of caterpillars have been reported to respond to sound, but there has been limited formal study of what sounds they hear, how they hear them, and how they respond to them. Here we report on hearing in caterpillars of the Monarch butterfly (Danaus plexippus). Fourth and fifth instar caterpillars respond to sounds by freezing, contracting, and flicking their thorax in a vertical direction. Behavioural responses were evoked by sound frequencies between 50 and 900 Hz, with best sensitivity at 100-200 Hz. The lowest mean threshold was 79 dBSPL (particle velocity 605 µm/s) at 150 Hz. When presented with a repeated 200 Hz sound tone, caterpillars habituate by no longer responding. A series of ablation experiments confirmed that the primary sensory receptors are a pair of long hairs, called trichoid sensilla, located on the upper prothorax. These sensilla are ∼450 µm long, rest in a socket, and are innervated by a single bipolar sensory neuron. Removal of these setae reduced responses significantly compared to controls. Other setae contributed minimally to hearing in response to 200 Hz tones, and tubercles and prothoracic shields played no apparent role in sound reception. We propose that hearing functions to prevent attacks by aerial insect predators and parasitoids, which produce flight sounds in the frequency range of the caterpillars’ sensitivity. This research lays the foundation for further investigations on the function and evolution of hearing in caterpillars, and has significance for conservation of threatened monarch butterfly larvae living near noisy urban environments and roadways.