The Ear of Dionysius (Italian: Orecchio di Dionisio) is an artificial limestone cave carved out of the Temenites hill in the city of Syracuse, on the island of Sicily in Italy.

The Ear of Dionysius was most likely formed out of an old limestone quarry. It is 23 metres high and extends 65 metres back into the cliff. Horizontally it bends in an approximate 's' shape; vertically it is tapered at the top like a teardrop. Because of its shape the Ear has extremely good acoustics, making even a small sound resonate throughout the cave.

The name of the cave was coined in 1586 by the painter Michelangelo Merisi (Caravaggio). It refers to the tyrant Dionysius of Syracuse. According to legend (possibly one created by Caravaggio), Dionysius used the cave as a prison for political dissidents, and by means of the perfect acoustics eavesdropped on the plans and secrets of his captives.

Because of its reputation for acoustic flawlessness, the Ear of Dionysius has also come to refer to a type of ear trumpet that has a flexible tube. The term 'Ear of Dionysius' can also refer to surveillance, specifically that for political gain.