Originally named Bŏvĭā́num, it was settled by the 7th century BC. As the capital of the Pentri, a tribe of the Samnites, it played a major role in the Samnite Wars, as well as in the Social War, when it was a temporary capital (89 BC). It was sacked by Sulla.

It was colonized under both the triumvirates, and by Vespasian, who settled veterans of Legio XI Claudia (whence the name Bovianum Undecumanorum), and remained an important centre into late antiquity.

After the Lombard conquest, the deserted area was given to a group of Bulgars, who circa 662 fled from the Avars and sought refuge with the Lombards. Bojano became a seat of a gastaldate. The Bulgars also settled in nearby Sepino and Isernia. Paul the Deacon in his Historia Langobardorum writing after the year 787 says that in his time Bulgars still inhabited the area, and that even though they speak "Latin", "they have not forsaken the use of their own tongue". In later times they had evidently become completely assimilated.

After two centuries marked by Saracen attacks, in the mid-11th century Bojano was conquered by the Hauteville Normans, becoming a fief of Raoul de Moulins, a companion to Robert Guiscard. The city became a county capital.

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