The first actual documentary evidence of the existence of the town is found only the Middle Ages, when Arbus, according to the first volume of the "Geographical, Historical, Statistical and Commercial Dictionary of the States of His Majesty King of Sardinia" (in Italian, "Dizionario geografico - storico - statistico -commerciale degli Stati di S. M. il Re di Sardegna"), was the capital, or rather part of the curatory or department, of the Kingdom of Colostrài that, as stated by the same source, was one of the departments of the ancient Kingdom of Arborea in Sardinia. According to these sources, it is as a result wise to say that Arbus was already existing in the Middle Ages but, while the Sardinian Kingdom of Arborea has a 500-year-long history, Arbus has not found yet its place in this long time span.