Arechis II (also Aretchis, Arichis, Arechi or Aregis) (born c. 734 – died 26 August 787) was a Duke of Benevento, in Southern Italy. He sought to expand the Beneventos' influence into areas of Italy that were still under Byzantine control, but he also had to defend against Charlemagne, who had conquered northern Italy.

Arechis was descended from the Lombards, who had invaded the Italian peninsula in the late sixth century. The Lombards established their kingdom in northern Italy. Its capital was at Pavia, and it also included two independent southern duchies—the Spoleto and Benevento.

Arechis was the son of Duke Liutprand, whom he succeeded in 758. Arechis continued to use the title Duke of Benevento until the Lombard kingdom fell to Charlemagne in 774. Arechis adopted the title Prince of Benevento after the fall of the kingdom—probably as a gesture of independence—and continued to use it until his death in 787.

Around 757, Arechis married Adelperga, a daughter of the northern Lombard king Desiderius. Arechis established friendly—but largely independent—relations with Desiderius. This lasted until 774, when the Lombard kingdom fell to the Franks and Desiderius was deposed. The Beneventan couple produced five children: three boys and two girls. ^{[citation needed]}

By eighth century standards, Adelperga and Arechis were notable patrons of culture. Adelperga commissioned the major Lombard writer Paul the Deacon to produce his Historia Romana, a textbook of Roman history which was widely used during the rest of the Middle Ages. Some historians also argue that they commissioned Paul's more famous Historia gentis Langobardorum although this is uncertain. Arechis arranged for the transfer of saints' relics to the newly commissioned church of Santa Sofia in Benevento. The church still survives, albeit heavily restored. It is decorated with rare eighth-century frescoes.

Arechis also put resources into building projects in Salerno, including a new palace and castle. Salerno became an increasingly important port city during Arechis' reign. Major southern monasteries, such as Montecassino and San Vincenzo al Volturno also received substantial donations from him.

Arechis was Duke when Charlemagne conquered the north Italian Lombard kingdom in 774. Although Arechis refused to submit when Charlemagne made himself king of the Lombards, Benevento was left largely unmolested. That same year Arechis adopted the title of "prince of Benevento". He also issued a handful of laws. Both these acts were probably intended as a gesture of defiance against Charlemagne—until this time only the Lombard kings had enacted laws. Since Charlemagne now styled himself king of the Lombards, Arechis was effectively rejecting Charlemagne's right to this title.

In November 774, immediately after being solemnly crowned Prince, Arechi II decided to send members of the Cortisani and Baccari families to occupy the middle area of the Biferno river, naming them as gestalds, the official royal authority in the area.