The Decretum Gratiani or Concordia discordantium canonum (in some manuscripts Concordantia discordantium canonum) is a collection of Canon law compiled and written in the 12th century as a legal textbook by the jurist known as Gratian. It forms the first part of the collection of six legal texts, which together became known as the Corpus Juris Canonici. It retained legal force in the Roman Catholic Church until Pentecost 1918, when a revised Code of Canon Law (Codex Iuris Canonici) promulgated by Pope Benedict XV on 27 May 1917 obtained the Force of Law.

Gratian was a canon lawyer from Bologna. He flourished in the mid 12th century. Little else is known about his biography.

He is sometimes incorrectly referred to as Franciscus Gratianus, Johannes Gratianus, or Giovanni Graziano. For a long time he was believed to have been born at the end of the 11th century, at Chiusi in Tuscany. He was said to have become a monk at Camaldoli and then he taught at the monastery of St. Felix in Bologna and devoted his life to studying canon law, but contemporary scholarship does not attach credibility to these traditions. Since the 11th century, Bologna had been the centre of the study of canon law, as well as of civil law, after the Corpus Juris Civilis was rediscovered in western Europe. Gratian's work was an attempt, using early scholastic method, to solve seemingly contradictory canons from previous centuries. Gratian quoted a great number of authorities, including the Bible, papal and conciliar legislation, church fathers such as Augustine of Hippo, and secular law in his efforts to reconcile the canons. Gratian found a place in Dante's Paradise among the doctors of the Church, and he was long acclaimed as "Father of Canon law", a title he shares with his successor Raymond of Penyafort.