Yves Marie-Joseph Congar OP (French pronunciation: [iv maʁi ʒozɛf kɔ̃ɡaʁ]; 13 April 1904 – 22 June 1995) was a French Dominican friar, priest, and theologian. He is perhaps best known for his influence at the Second Vatican Council and for reviving theological interest in the Holy Spirit for the life of individuals and of the church. He was created a cardinal of the Catholic Church in 1994.

Congar was born in Sedan in northeast France in 1904. His father Georges Congar was a bank manager. Congar's hometown was occupied by German military forces for much of World War I, and his father was among the men deported to Lithuania. Upon the urging of his mother, Lucie Congar née Desoye (called "Tere" by Yves throughout his life), Congar recorded the occupation in an extensive series of illustrated diaries which were later published. They provide a unique historical insight into the war from a child's point of view.

Encouraged by a local priest Daniel Lallement, Congar entered the diocesan seminary. Moving to Paris in 1921, he had Jacques Maritain among his philosophy teachers and the Dominican theologian Reginald Garrigou-Lagrange as a retreat master.

After a year of compulsory military service (1924–1925), which Congar spent in the Rhineland, in 1925 he joined the Dominican Order at Amiens, where he took Marie-Joseph as his name in religion. Towards the end of his theological studies from 1926 to 1931 at Le Saulchoir, the Dominican theologate which was then located in Kain-la-Tombe, Belgium, and focused on historical theology, Congar was ordained a priest on 25 July 1930 by Luigi Maglione, nuncio in Paris. In 1931 Congar defended his doctoral dissertation written at Le Saulchoir, on the unity of the Church.

Congar was a faculty member at Le Saulchoir from 1931 to 1939, moving with the Institution in 1937 from Kain-la-Tombe to Étiolles near Paris. In 1932 he began his teaching career as Professor of Fundamental Theology, conducting a course on ecclesiology. Congar was influenced by the Dominicans Ambroise Gardeil and Marie-Dominique Chenu, by the writings of Johann Adam Möhler, and by his ecumenical contacts with Protestant and Eastern Orthodox theologians. Congar concluded that the mission of the church was impeded by what he and Chenu termed "baroque theology."

In 1937 Congar founded the Unam Sanctam series, addressing historical themes in Catholic ecclesiology. These books called for a "return to the sources" to set theological foundations for ecumenism, and the series would eventually run to 77 volumes. He wrote for a wide variety of scholarly and popular journals, and published numerous books.

During World War II Congar was drafted into the French army as a chaplain with the rank of lieutenant. He was captured and held from 1940 to 1945 as a prisoner of war by the Germans in Colditz and Lübeck's Oflag, after repeated attempts to escape. Later he was made a Knight (Chevalier) of the French Legion of Honour, and awarded the Croix de Guerre. In addition he was awarded the Médaille des Évadés for his numerous escape attempts.

After the war, Congar continued to teach at Le Saulchoir, which had been returned to France, and to write, eventually becoming one of the most influential theologians of the 20th century on the topic of the Catholic Church and ecumenism.