Adelchis (died May 878) was the son of Radelchis I, Prince of Benevento, and successor of his brother Radelgar in 854.

It was given to Adelchis to preserve the ancient principality and its independence in the face of repeated assaults by the Saracens from the south, the Emperor Louis II from the north, and Byzantine Langobardia to the east.

The response of Adelchis to this action was to imprison and rob the emperor while he was staying in the princely palace at Benevento in August—a treachery lamented in a contemporary poem, the Rythmus de captivitate Ludovici imperatoris.

A month later, the Saracens had landed with a new invasive force and Adelchis released Louis to lead the armies against it.

Adelchis forced Louis to vow never to reenter Benevento with an army or to take revenge for his detention.