Theudebert I (French: Thibert or Théodebert) (c. 503 – 547 or 548) was the Merovingian king of Austrasia from 533 to his death in 548. He was the son of Theuderic I and the father of Theudebald.
Most of what we know about Theudebert comes from the Histories or History of the Franks written by Gregory of Tours in the second half of the sixth century. In addition, we have diplomatic correspondence composed at the Austrasian court (known as the Austrasian Letters), the poems of Venantius Fortunatus, an account from Procopius' work and a small number of other sources.
During his father's reign, the young Theudebert had shown himself to be an able warrior. In about 516 he defeated a Danish army under King Chlochilaich (Hygelac of Beowulf) after it had raided northern Gaul. His reputation was further enhanced by a series of military campaigns in Septimania against the Visigoths.
Upon his father's death, Theudebert had to fight both his uncles Childebert and Clotaire to inherit his father's kingdom. In the end, his military prowess persuaded Childebert to abandon the dispute and adopt Theudebert as his heir. Together they campaigned against Clotaire but sued for peace after their armies were hit by storm.
After relations between the Frankish kings had settled down, Theudebert found himself embroiled in the Gothic War started when the Byzantine Emperor Justinian I attempted to subdue the Ostrogoths in Italy. Justinian saw Theudebert as an ideal ally: Austrasian lands flanked the Ostrogoths in northern Italy. The emperor paid Theudebert handsomely for his assistance, but Theudebert proved an untrustworthy ally. The Frankish armies saw the Italian conflict as an opportunity for plunder and a chance to exert their own claims to northern Italy. In the event the Byzantines were forced to fight the Franks as much as the Ostrogoths.