Sigebert I was the Germanic king of Austrasia from the death of his father in 561 to his own death. He was the third surviving son out of four of Clotaire I and Ingund. His reign found him mostly occupied with a successful civil war against his half-brother, Chilperic.
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, with its capital at Rheims, to which he added further territory on the death of his brother, Charibert, in 567 or 568; Charibert himself had received the kingdom centred on Paris; Guntram received the Kingdom of Burgundy with its capital at Orléans; and the youngest son, the aforementiond Chilperic, received Soissons, which became Neustria when he received his share of Charibert's kingdom. Incursions by the Avars, a fierce nomadic tribe related to the Huns, caused Sigebert to move his capital from Rheims to Metz. He repelled their attacks twice, in 562 and c. 568.
In 573, Sigebert took possession of Poitiers and Touraine, and conquered most of his brother Chilperic's 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-en-Artois, he was struck down by two assassins working for Fredegund.
He was succeeded by his son Childebert under the regency of Brunhilda. Brunhilda and Childebert quickly put themselves under the protection of Guntram, who eventually adopted Childebert as his own son and heir. With Brunhilda he had two daughters: Ingund and Chlodosind.