The Tyrrhenian Sea (Corsican: Mari Tirrenu, French: Mer Tyrrhénienne, Italian: Mare Tirreno, Neapolitan: Mar Tirreno, Sardinian: Mare Tirrenu, Sicilian: Mari Tirrenu, Latin: Mare Tyrrhenum) is part of the Mediterranean Sea off the western coast of Italy.

The sea is bounded by Corsica and Sardinia (to the west), Tuscany, Lazio, Campania, Basilicata and Calabria (to the east) and Sicily (to the south).

The maximum depth of the sea is 3,785 metres (12,418 ft).

The Tyrrhenian Sea is situated near where the African and European Plates meet; therefore mountain chains and active volcanoes such as Mount Marsili are found in its depths. The eight Aeolian Islands and Ustica are located in the southern part of the sea, north of Sicily.

The International Hydrographic Organization defines the limits of the Tyrrhenian Sea as follows:

There are four exits from the Tyrrhenian Sea (north to south):

The Tyrrhenian Sea is divided into two basins (or plains), the Vavilov plain and the Marsili plain. They are separated by the undersea ridge known as the Issel Bridge, after Arturo Issel.^{[dead link]}