American Protestant theologian and educator who was considered a leading authority on ethics and U.S. church history. He was a foremost advocate of theological existentialism.

The younger brother of the theologian Reinhold Niebuhr, Helmut was educated at Elmhurst (Ill.) College, Eden Theological Seminary (St. Louis, Mo.), Washington University, Yale Divinity School, and Yale University, where he was one of the first students to receive a Ph.D. in religion (1924). Ordained a pastor of the Evangelical and Reformed Church in 1916, he taught at Eden Theological Seminary (1919–22; 1927–31) and also served as president of Elmhurst College (1924–27). From 1931 he taught theology and Christian ethics at Yale Divinity School.

Influenced by Karl Barth, Søren Kierkegaard, and Ernst Troelsch, Niebuhr advocated historical criticism of religious beliefs, urging that church teachings be interpreted in a way to make them meaningful in contemporary culture. His views on theological existentialism allowed for relative interpretations of revelation and values within the framework of a monotheistic faith. Niebuhr argued that there is a correlation between religious belief and secular culture and that churches must account for the social context of their existence.

Among the most important works published by Niebuhr are The Social Sources of Denominationalism (1929), The Kingdom of God in America (1937), The Meaning of Revelation (1941), The Purpose of the Church and Its Ministry (1954), Radical Monotheism and Western Culture (1960), and The Responsible Self (1963).