The Decretum Gratiani, also known as the Concordia discordantium canonum or Concordantia discordantium canonum or simply as the Decretum, is a collection of Catholic 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 was used as the main source of law by canonists of the Catholic Church until the Decretals, promulgated by Pope Gregory IX in 1234, obtained legal force, after which it was the cornerstone of the Corpus Juris Canonici, in force until 1917.

In the first half of the 12th century Gratian, clusinus episcopus, probably a jurist of the ecclesiastical forum and a teacher, rubricator at the monastery of Saints Nabor and Felix (according to the Bolognese Odofredus Denariis [13th century]) and starting from the 18th century believed to have been a Camaldolese monk, composed the work he called Concordia discordantium canonum, and others titled Nova collectio, Decreta, Corpus juris canonici, or the more commonly accepted name, Decretum Gratiani, a living text, characterized by multiple editorial stages. He did this to obviate the difficulties which beset the study and the forensic application of practical, external theology (theologia practica externa), i.e., the study and the forensic use of canon law. In spite of its great reputation and wide diffusion, the Decretum has never been recognized by the Church as an official collection.

The so-called vulgata or vulgate version (an advanced editorial stage) of the Decretum is divided into three parts (ministeria, negotia, sacramenta).

Each distinction or question contains dicta Gratiani, or maxims of Gratian, and canones. Gratian himself raises questions and brings forward difficulties, which he answers by quoting auctoritates, i. e. canons of councils, decretals of the popes, texts of the Scripture or of the Fathers. These are the canones; the entire remaining portion, even the summaries of the canons and the chronological indications, are called the maxims or dicta Gratiani.

Many auctoritates have been inserted in the Decretum by authors of a later date. These are the Paleae, so called from Paucapalea, the name of the principal commentator on the Decretum. The Roman revisers of the 16th century (1566–1582) corrected the text of the "Decree" and added many critical notes designated by the words Correctores Romani.

The Decretum is cited by referring to the larger units of the distinction or the cause and question, and then the specific canon or dictum. For clarity, the distinctions of Causa 33, quaestio 3 of the second part are referred to as De penitentia (or De pen.), while the distinctions of the third part are referred to as De consecratione (or De cons.). The Part is usually not included, as the citation form is different for each.

Citation styles for the Decretum have changed over time and can generally be categorised under the modern, obsolescent, and obsolete forms.

This form, common since the twentieth century, cites all units in Arabic numerals, from largest unit to smallest unit.