Sigisbert was the son of Ragnetrude and Dagobert I. Baptised by Saint Armand about a month after his birth in 630 AD. he would have, according to legend, answered aloud “Amen” to the blessings.

Made King of Austrasia by his father Dagobert I in 633 AD. Sigebert  III was governed first by his regents, Bishop Chunibert of Cologne and Duke Adalgisa; then, on Dagobert’s death, by Chunibert and Pippin I, the mayor of the palace and finally by Pippin’s son, Grimoald, mayor of the palace from 642 or 656 AD

Sigisbert was proclaimed King of Austrasia at the age of 3. 

Very pious, he favoured the progression of Christianity by sending missionaries to areas of his Kingdom that had still remained pagan. 

According to tradition Sigisbert III is at the origin of 12 to 20 monasteries. We know for sure that he founded in the Ardennes those of Cugnon, Malmedy and Stavelot.

He married Princess Mathilde of Burgundy who in 651AD. gave him a son, Dagobert II. 

In the early 640s, because of a power struggle between Otto, Sigebert’s tutor, and Grimoald, Thuringia succeeded in gaining effective autonomy at Austrasian expense. 

At the end of 649 AD. or perhaps at the beginning of 650 AD. Amandus, the bishop of Maastricht, received a papyrus roll containing the acts of the council held in the Lateran to condemn monothelitism from Pope Martin I. In a long letter, Pope Martin asked Amandus and the Frankish King of Austrasia, Sigebert III, to support him against the Byzantine emperor.

The Life of Saint Eligius of Noyon attest to strong bonds which existed between Christian clergy in the West and the Byzantine Emperor in the East.

After a reign of 23 years Sigisbert III died on February 1, 656 AD. he was murdered at instigation of the mayor of the palace Grimoald.

According to his will, his body was placed in the Crypt of the Church of the Abbey of Saint-Martin-Devant-Metz.