The Apennines or Apennine Mountains (; Greek: Ἀπέννινα ὄρη or Ἀπέννινον ὄρος; Latin: Appenninus or Apenninus Mons – a singular with plural meaning; Italian: Appennini [appenˈniːni]) are a mountain range consisting of parallel smaller chains extending c. 1,200 km (750 mi) along the length of peninsular Italy. In the northwest they join with the Ligurian Alps at Altare. In the southwest they end at Reggio di Calabria, the coastal city at the tip of the peninsula. Since 2000 the Environment Ministry of Italy, following the recommendations of the Apennines Park of Europe Project, has been defining the Apennines System to include the mountains of north Sicily, for a total distance of 1,500 kilometres (930 mi). The system forms an arc enclosing the east side of the Ligurian and Tyrrhenian Seas.

The Apennines conserve some intact ecosystems that have survived human intervention. In these are some of the best preserved forests and montane grasslands in Europe, now protected by national parks and, within them, a high diversity of flora and fauna. These mountains are one of the last refuges of the big European predators such as the Italian wolf and the Marsican brown bear, now extinct in the rest of Central Europe.

The mountains lend their name to the Apennine peninsula that forms the major part of Italy. They are mostly verdant, although one side of the highest peak, Corno Grande, is partially covered by Calderone glacier, the only glacier in the Apennines. The eastern slopes down to the Adriatic Sea are steep, while the western slopes form foothills on which most of peninsular Italy's cities are located. The mountains tend to be named from the province or provinces in which they are located; for example, the Ligurian Apennines are in Liguria.