Pope Sixtus III (died 18 August 440) was the head of the Catholic Church from 31 July 432 to his death in 440.
The name of Sixtus is often connected with a great building boom in Rome: Santa Sabina on the Aventine Hill was dedicated during his pontificate and he built Santa Maria Maggiore, whose dedication to Mary the Mother of God reflected his acceptance of the Ecumenical council of Ephesus which closed in 431. At that council, the debate over Christ's human and divine natures turned on whether Mary could legitimately be called the "Mother of God" or only "Mother of Christ". The council gave her the Greek title Theotokos (literally "God-bearer", or "Mother of God"), and the dedication of the large church in Rome is a response to that.
Prior to being made Pope, Sixtus was a patron of Pelagius, who was later condemned as a heretic.