John Gresham Machen (July 28, 1881–January 1, 1937) was an early 20th-century American Presbyterian scholar and educator specializing in the New Testament. He served as the New Testament Professor at Princeton Seminary from 1906 to 1929, where he opposed modernist theology and established Westminster Theological Seminary as a more traditional alternative. As the Northern Presbyterian Church increasingly dismissed conservative efforts to uphold the Westminster Confession, Machen led a group of conservatives to create the Orthodox Presbyterian Church. In the mid-1920s, after the Presbyterian Church in the United States of America (PCUSA) rejected his views and opted to transform Princeton Seminary into a liberal institution, Machen spearheaded the establishment of Westminster Seminary in Philadelphia in 1929, where he taught New Testament until his passing. His ongoing resistance to liberalism within the foreign missions of his denomination in the 1930s resulted in the founding of the Independent Board for Presbyterian Foreign Missions in 1933. The trial, conviction, and suspension of members from the Independent Board, including Machen, in 1935 and 1936 justified the creation of the OPC in 1936.

Machen is regarded as the last of the prominent theologians from Princeton, who had developed Princeton theology—a conservative and Calvinist variant of Evangelical Christianity—since the seminary’s inception in the early 19th century. While he can be compared to notable Princeton theologians like Archibald Alexander, Charles Hodge, A. A. Hodge, and B. B. Warfield, he was not a theology lecturer (his expertise was in the New Testament) and never held the position of principal at the seminary. Machen’s impact is still evident today through the institutions he founded: Westminster Theological Seminary, the Independent Board for Presbyterian Foreign Missions, and the Orthodox Presbyterian Church. Additionally, his textbook on basic New Testament Greek continues to be utilized in various seminaries, including those affiliated with PCUSA.