Pope Sixtus III (died 18 August 440) was Pope of the Catholic Church from 31 July 432 to his death in 440. His ascension to the papacy is associated with a period of increased construction in the city of Rome. His feast day is celebrated by Catholics on March 28th.
Peter Brown says that prior to being made Pope, Sixtus was a patron of Pelagius, who was later condemned as a heretic, although Butler disagrees and attributes the charge to Garnier. Nicholas Weber also disputes this, "...it was probably owing to his conciliatory disposition that he was falsely accused of leanings towards these heresies."
Sixtus was consecrated Pope on 31 July, 432. He attempted to restore peace between Cyril of Alexandria and John of Antioch. He also defended the rights of the Pope over Illyria and the position of the archbishop of Thessalonica as head of the local Illyrian church against the ambition of Proclus of Constantinople.
His name is often connected with a great building boom in Rome: Santa Sabina on the Aventine Hill was dedicated during his pontificate. He built the Liberian Basilica as 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.
His feast is kept on 28 March.