Sigebert III was the Merovingian king of Austrasia from 633 to his death around 656. He was described as the first Merovingian roi fainéant —do-nothing king—, in effect the mayor of the palace ruling the kingdom throughout his reign. However he lived a pious Christian life and was later sanctified, being remembered as Saint Sigebert of Austrasia in the Roman Catholic Church and Eastern Orthodox Church.

Austrasia was a territory which formed the north-eastern section of the Kingdom of the Franks from the 6th to 8th centuries, ruled by the Frankish Merovingian and Carolingian dynasties during the Early Middle Ages. It was centred on the Meuse, Middle Rhine, and the Moselle rivers, and was the original territory of the Franks, including both the so-called Salian Franks and Ripuarian Franks, which Clovis I, King of the Franks (481–511) conquered after first taking control of the bordering part of Roman Gaul, which is sometimes described in this period as Neustria.