Louis the Pious (German: Ludwig der Fromme; 16 April 778 – 20 June 840), also called the Fair, and the Debonaire, was King of the Franks and co-emperor with his father, Charlemagne, from 813. He was also King of Aquitaine from 781. As the only surviving son of Charlemagne and Hildegard, he became the sole ruler of the Franks after his father's death in 814, a position which he held until his death, save for the period 833–34, during which he was deposed.

During his reign in Aquitaine, Louis was charged with the defence of the empire's southwestern frontier. He conquered Barcelona from the Emirate of Córdoba in 801 and asserted Frankish authority over Pamplona and the Basques south of the Pyrenees in 812. As emperor he included his adult sons, Lothair, Pepin, and Louis, in the government and sought to establish a suitable division of the realm among them. The first decade of his reign was characterised by several tragedies and embarrassments, notably the brutal treatment of his nephew Bernard of Italy, for which Louis atoned in a public act of self-debasement.

In the 830s his empire was torn by civil war between his sons, only exacerbated by Louis's attempts to include his son Charles by his second wife in the succession plans. Though his reign ended on a high note, with order largely restored to his empire, it was followed by three years of civil war. Louis is generally compared unfavourably to his father, though the problems he faced were of a distinctly different sort.