Roman jurist whose writings became authoritative in the late Roman Empire. The Law of Citations (426), issued by the eastern Roman emperor Theodosius II, named Gaius one of five jurists (the others were Papinian, Ulpian, Modestinus, and Paulus) whose doctrines were to be followed by judges in deciding cases. The Institutiones (“Institutes”) of the Byzantine emperor Justinian I (reigned 527–565), which were intended to supersede Gaius's treatise of the same name, were modeled on the older work in style and content, and numerous passages were copied verbatim.

Gaius's full name and most of his personal history are unknown. Besides the Institutiones, he wrote a commentary on the Law of the Twelve Tables (Twelve Tables, Law of the) (laws said to have been enacted about 450 BC, under the Roman monarchy), a treatise on the edicts of Roman magistrates, and several other works. His writings show an interest in legal history, Greek philosophy, and the classification of laws.

The Institutiones of Gaius, written about AD 161, comprise four books. The first concerns the legal status of persons; the second and third, property rights, including inheritance; and the fourth, forms of legal actions. The text was lost until 1816, when a manuscript, probably of the 5th century, was discovered at Verona, Italy. It was deciphered with great difficulty, because writings of St. Jerome (Jerome, Saint) had been superimposed on Gaius's words. It is the only classical law book to have survived nearly complete and unchanged during the time of Justinian.