John Gresham Machen (1881-1937) was an influential American Presbyterian theologian in the early 20th century. He was the Professor of New Testament at Princeton Seminary between 1915 and 1929, and led a conservative revolt against modernist theology at Princeton and formed Westminster Seminary as a more orthodox alternative. This split was irreconcilable, and Machen led others to form the Orthodox Presbyterian Church.

Machen is considered to be the last of the great Princeton Theologians who had, since the formation of the college in the early 19th century, developed Princeton Theology - a conservative and Calvinist form of Evangelical Christianity. Although Machen can be compared to the great Princeton Theologians ( Archibald Alexander, Charles Hodge, A. A. Hodge and B. B. Warfield) he was neither a lecturer in theology (he was a New Testament scholar) nor did he ever become the seminary's principal.

Machen's influence can still be felt today through the existence of both institutions that he founded - Westminster Seminary and the Orthodox Presbyterian Church.