Messenia (/məˈsiːniə, -ˈsiːnjə/; Greek: Μεσσηνία Messinia, pronounced [mesiˈnia]) is a regional unit (perifereiaki enotita) in the southwestern part of the Peloponnese, one of 13 regions into which Greece has been divided by the Kallikratis plan, implemented 1 January 2011. Before 2011 Messenia was a nomos (prefecture). The capital and the biggest city of Messenia in either case has been the city of Kalamata.
Messenia borders on Elis to the north, Arcadia to the northeast, and Laconia to the southeast. The Ionian Sea lies to the west, and the Gulf of Messenia to the south. The most important mountain ranges are the Taygetus in the east, the Kyparissia mountains in the northwest and the Lykodimo in the southwest. The main rivers are the Neda in the north and the Pamisos in central Messenia.
Off the south coast of the southwesternmost point of Messenia lie the Messenian Oinousses islands. The largest of these are Sapientza, Schiza and Venetiko. The small island Sphacteria closes off the bay of Pylos. All these islands are virtually uninhabited.