lawyer, law teacher at Perugia, and chief among the postglossators, or commentators (legal glossator), a group of northern Italian jurists who, from the mid-14th century, wrote on the Roman (civil) law (Roman law). Their predecessors, the glossators (legal glossator), had worked at Bologna from about 1125.

Bartolus studied law at the universities of Perugia and Bologna and held the chair of law at Perugia from 1343 onward. He and his colleagues used the Corpus juris civilis (Justinian, Code of) (“Body of Civil Law”; also known as the Code of Justinian (Justinian, Code of)) of the 6th-century Byzantine emperor Justinian I and the work of the glossators thereon, together with Roman civil law, as a foundation from which to derive broad legal principles that could be used to solve contemporary problems in 14th-century Europe. Through this process Bartolus wrote several extremely influential legal doctrines, particularly those on the governmental authority of city-states and the rights of individuals and corporate bodies within them. These and other of his principles became the common law of Italy and were also recognized as law in Spain, Portugal, and Germany. Bartolus's commentaries on the Corpus juris civilis were sometimes accorded an authority equal to that of the code itself.