See Sigeberht I of Essex for the Saxon ruler by that name

Sigebert I (535-575) was a Frankish King, one of the sons of Clotaire I and Ingund . He successfully pursued a civil war against his half brother, Chilperic I.

When Clotaire I died in 561, his kingdom was divided, in accordance with Frankish custom, among his four sons; Sigebert became king of the northeastern portion, known as Austrasia, to which he added further territory on the death of his brother, Charibert, in 567 or 568. Incursions by the Avars, a fierce nomadic tribe related to the Huns, caused him to move his capital from Reims to Metz. He repelled their attacks twice, in 562 and c. 568.

About 567 he married Brunhilda, daughter of the Visigothic king Athanagild, whose other daughter, Galswintha, married Chilperic I. When Chilperic had Galswintha murdered in order to marry Fredegund, Sigebert sought revenge. The two brothers had already fought each other, but their hostility now elevated into a long and bitter war that was continued by the descendants of both.

Sigebert defeated Chilperic and conquered most of his kingdom. Chilperic then hid in Tournai. But at Sigebert's moment of triumph, when he had just been declared king by Chilperic's subjects at Vitry , he was struck down by two assassins working for Fredegund.