The Castle of São Jorge (Portuguese: Castelo de São Jorge; Portuguese pronunciation: [kɐʃˈtɛlu dɨ sɐ̃w̃ ˈʒɔɾʒ(ɨ)]) is a Moorish castle that occupies a commanding position overlooking the city of Lisbon, the capital of Portugal, and the broad Tagus River (Portuguese: Rio Tejo) beyond. The strongly fortified citadel, which, in its present configuration, dates from medieval times, is located atop the highest hill in the historic center of the city. The castle is one of the main historical and touristic sites of Lisbon.

Although the first fortifications on this Lisbon hilltop are known to be no older than the 2nd century BC, archaeological research has shown that humans have occupied the site since the 6th century BC, and possibly earlier. The hill was employed in early times by indigenous Celtic tribes, and others, probably Phoenicians, Greeks, and Carthaginians, have also left their cultural footprints there. Afterwards, Roman, Suebic, Visigothic, and Moorish settlers lived where the castle now stands.