Louis the Pious (778 – 20 June 840), also called the Fair, and the Debonaire, is the King of Aquitaine from 781.

He is also King of the Franks and co-Emperor (as Louis I) with his father, Charlemagne, from 813.

As the only surviving adult son of Charlemagne and Hildegard, he becomes the sole ruler of the Franks after his father's death in 814, a position which he holds until his death, save for the period 833–34, during which he is deposed.

During his reign in Aquitaine, Louis is charged with the defense of the Empire's southwestern frontier.

He conquers Barcelona from the Muslims in 801 and asserts Frankish authority over Pamplona and the Basques south of the Pyrenees in 812.

As emperor, he includes his adult sons, Lothair, Pepin, and Louis, in the government and seeks to establish a suitable division of the realm among them.

The first decade of his reign is characterized by several tragedies and embarrassments, notably the brutal treatment of his nephew Bernard of Italy, for which Louis atones in a public act of self-debasement.

In the 830s, his empire is 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 ends on a high note, with order largely restored to his empire, it is followed by three years of civil war.

Louis is generally compared unfavorably to his father, though the problems he faced were of a distinctly different sort.