Tyrrhenian Sea (Italian Mare Tirreno), arm of the Mediterranean Sea, partially enclosed by the Italian Peninsula and the islands of Sicily, Sardinia, and Corsica. The Ligurian Sea lies to the north. The Tyrrhenian Sea is connected to the Ionian Sea by the Strait of Messina. Main ports are Naples, on the mainland, and Palermo, on Sicily. Area 155,000 sq km (59,800 sq mi).