Sixtus III (d. 440) was pope from July 31, 432 to August 18, 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 be called the "Mother of Jesus" as a human only or the "Mother of Christ" as both God and Man. The council gave her the Greek title Theotokos ("God-bearer"), and the dedication of the large church in Rome is a response to that.

One of his main concerns was in restoring peace between Cyril of Alexandria and the Syrians.

He also maintained the rights of the pope over Illyria and the position of the archbishop of Thessalonica as head of the Illyrian church.