Sigebert III (630?-660) was a Frankish King of Austrasia who began his rule in around 635 and ended it with his death in approximately February 660. He was the eldest son of Dagobert I.

To satisfy the Austrasian aristocracy, who exercised a certain autonomy, Sigebert's father gave him the kingdom of Austrasia although it remained part of the larger Frankish realm. On the death of Dagobert, Sigebert ruled Austrasia independently, and free from any subjection to Neustria.

He tried in vain to add Thuringia to his kingdom, but was defeated by Duke Radulph in 640. It was under his reign that the mayor of the palace began to play the most important role in the political life of Austrasia.

Grimaud, the son of Pepin I, managed to convince the king to adopt his son Childebert. When Sigebert finally had a son of his own, the future Dagobert II, the mayor of the palace felt threatened, and on the death of Sigebert he exiled the young Dagobert to Ireland. Sigebert's remains, defiled during the French Revolution, are preserved in the cathedral at Nancy.