Louis II the Younger (825 – 12 August 875) was the King of Italy from 844 and then Emperor from 855 until his death.

He was the eldest son of the Emperor Lothair I and Ermengarde of Tours. He was designated King of Italy in 839 and took up his residence in that country and was crowned king at Rome by Pope Sergius II on 15 June 844. He at once claimed the rights of an emperor in the city, which claim was decisively rejected; but in 850 he was crowned joint emperor at Rome by Pope Leo IV, and soon afterwards, in 851, married Engelberga and undertook the independent government of Italy. He marched into the south of Italy in the year of his imperial coronation and compelled the rival dukes of Benevento, Radelchis I and Siconulf, to make peace. His mediation split the Lombard duchy and gave Radelchis his share with Benevento as his capital and gave Salerno as a principality independent to Siconulf. Radelchis, now pacified, had no need of his Saracen mercenaries and happily betrayed them to the emperor. Louis fell on them and they were massacred. He then quashed some accusations against Pope Leo and held a Diet at Pavia. He confirmed the usurping regent Peter as prince of Salerno in December 853, displacing the dynasty he had installed there three years earlier. On the death of his father in September 855, he became sole emperor.

The division of Lothair's dominions, by which he obtained no territory outside Italy, aroused his discontent, and in 857 he allied himself with Louis the German against his own brother Lothair, King of Lotharingia, and King Charles the Bald. But after Louis had secured the election of Pope Nicholas I in 858, he became reconciled with his brother, and received some lands south of the Jura mountains in return for assistance given to Lothair in his efforts to obtain a divorce from his wife, Teutberga.

In 863, on the death of his brother Charles, Louis received the kingdom of Provence, and in 864 came into collision with Pope Nicholas I over his brother's divorce. The archbishops, who had been deposed by Nicholas for proclaiming this marriage invalid, obtained the support of the emperor, who reached Rome with an army in February 864; but, having been seized with fever, he made peace with the pope and left the city.

In his efforts to restore order in Italy, Louis met with considerable success both against Italy's turbulent princes and against the Saracens who were ravaging southern Italy. In 866 he routed these invaders, but could not follow up his successes owing to the lack of a fleet. So in 869 he made alliance with the eastern emperor, Basil I, who sent him ships to assist in the capture of Bari, capital of a local Islamic emirate, which succumbed in 871.
Meanwhile his brother Lothair had died in 869, and owing to his detention in southern Italy, Louis failed to prevent the partition of Lotharingia between Louis the German and Charles the Bald. Some jealousy between Louis and Basil followed the victory at Bari, and in reply to an insult from the eastern emperor Louis attempted to justify his right to the title "emperor of the Romans."

He had withdrawn into Benevento to prepare for a further campaign when he was treacherously attacked in his palace, robbed and imprisoned by Adelchis, prince of Benevento, in August 871. The landing of fresh bands of Saracens compelled Adelchis to release his prisoner a month later, and Louis was forced to swear he would take no revenge for this injury, nor ever enter Benevento with an army. Returning to Rome, he was released from his oath, and was crowned a second time as emperor by Pope Adrian II on May 18, 872.

Then Louis won further successes against the Saracens, who were driven from Capua, but the emperor's attempts to punish Adeichis were not very successful. Returning to northern Italy, he died, near Ghedi, in what is now the province of Brescia, on August 12, 875, and was buried in the church of Basilica of Sant'Ambrogio at Milan, having named as his successor in Italy his cousin Carloman, son of Louis the German.