The Duchy of Alvito was a fiefdom of the Kingdom of Naples, in southern Italy.
The Cantelmo family, of French origins, had arrived in Italy in the 13th century around the time the Angevins conquered Naples (1266). From the Angevins the Cantelmo received several castles and fiefs around Alvito in what is now the Valcomino, dominated by the numerous fiefs of the powerful regional monasteries (such as Monte Cassino and San Vincenzo al Volturno), as well as of the rival family of the Counts of Aquino. Rostaino and his successors tried unsuccessfully to extend their control over the whole region in the 14th century. They sided with Queen Joan II of Naples but were defeated by the troops of Charles of Durazzo. In a document of 1384, Giacomo IV is mentioned as "lord of the lands of Alvito".
During the turmoil caused by the succession of Ladislaus of Naples, the Cantelmo sided with his rival, Louis II of Anjou. Rostainuccio ("little Rostaino") was defeated and captured by Jacopo Orsini at Pereto in Abruzzo in 1369, and Alvito was granted by Ladislaus to Andrea Tomacelli, brother of the Count of Sora and of Pope Boniface IX. Alvito was soon returned to the Cantelmo, and Giacomo V, Rostainuccio's son, was the first ruler of Alvito to bear the title count. He likely obtained the title as a dowry from his wife, Elisabetta d'Aquino. After Giacomo's death the county was expanded by his son Antonio, who acquired Gallinaro, Fontechiari, Arce, Popoli (this from his brother Francesco, who had died heirless), and other lands in the Abruzzo and Valcomino. Otherwise, Antonio's rule was troubled by the turmoil and succession crises of the Kingdom of Naples in the early 15th century, and he lost his lands repeatedly. His son Nicolò was able to establish his rule more firmly by backing the successful Alfonso V of Aragon in his conquests of 1443. Alfonso created Nicolò Duke of Sora, and in this capacity he stripped his brother Onofrio of the County of Popoli in Abruzzo. Nicolò also obtained the position of royal counsellor in 1452 and, just before his death, his title was raised to Duke of Alvito.