Chlothar I, sometime called "the Old" (French: le Vieux), (died c. December 561) also anglicised as Clotaire from the original French version, was a king of the Franks of the Merovingian dynasty and one of the four sons of Clovis I.

With his eldest brother Theuderic (c. 485 – 533/34) being the son of Clovis I and his first wife, Chlothar followed his two elder brothers Chlodomer (495–524) and Childebert I (496–558) as third surviving son of Clovis I and his second wife Queen Clotilde, lastly followed by their sister Clotilde (500–531). The name 'Chlothar' means "glory".

In 511, Chlothar I and his three brothers Theuderic, Chlodomer and Childebert inherited their shares of their father's kingdom. Chlothar spent most of his life in a campaign to expand his territories at the expense of his relatives and neighbouring realms in all directions.

His brothers avoided outright war by cooperating with Chlothar's attacks on neighbouring lands in concert or by invading lands when their rulers died. The spoils were shared between the participating brothers. By the end of his life, Chlothar had managed to reunite Francia by surviving his brothers and seizing their territories after they died. But upon his own death, the Kingdom of the Franks was once again divided between his own four surviving sons. A fifth son had rebelled and was killed, along with his family.

Chlothar's father, Clovis I, had converted to Nicene Christianity, but Chlothar, like other Merovingians, did not consider that the Christian doctrine of monogamy should be expected of royalty: he had five wives, more from political expediency, for the purpose of forming alliances, than for personal motives. Although at the instigation of his queens he gave money for several new ecclesiastical edifices, he was a less than enthusiastic Christian and succeeded in introducing taxes on ecclesiastical property.

Frankish customs of the day allowed for the practice of polygamy, especially among royalty. So it was not uncommon for a king to have multiple wives and several competing heirs upon his death. This was a major deviation from the monogamy of late Roman customs, influenced by the Church. Frankish rulers followed this practice mainly to increase their influence across larger areas of land in the wake of the Roman empire's collapse. The aim was to maintain peace and ensure the preservation of the kingdom by appeasing local leaders. In the Germanic tradition succession fell, not to sons, but to younger brothers, uncles, and cousins. But under Salic law, Clovis I instituted the custom of sons being the primary heirs in all respects. However, it was not a system of primogeniture, with the eldest son receiving the vast majority of an inheritance, rather the inheritance was split evenly between all the sons. Therefore, the greater Frankish Kingdom was often splintered into smaller sub-kingdoms.

According to Gregory of Tours, "The King Chlothar had seven sons of various women, namely: with Ingund he had Gunthar, Childeric, Charibert, Guntram, Sigebert, and a daughter named Chlothsind; of Aregund, sister of Ingund he had Chilperic; and of Chunsine he had Chram." Chlothar's first marriage was to Guntheuc, widow of his brother Chlodomer, sometime around 524. They had no children. His second marriage, which occurred around 532, was to Radegund, daughter of Bertachar, King of Thuringia, whom he and his brother Theuderic defeated. She was later canonized. They also had no children. His third and most successful marriage was to Ingund, by whom he had five sons and two daughters:

Chlothar likely had an illegitimate son named Gondovald with an unnamed woman, born sometime in the late 540s or early 550s. Since Chlothar had sown children all throughout Gaul this was not unlikely. The boy was given a literary education and allowed to grow his hair long, a symbol of belonging to royalty. Although Chlothar would offer no more aid or privilege to the boy, his mother took him to the court of Childebert, who recognized him as his nephew and agreed to keep him in court.