William Harry Marshner (born August 14, 1943) is an American retired Emeritus Professor of Theology at Christendom College in Front Royal, Virginia.

He has written extensively on ethics and Thomism, and is most widely read as the co-author of Cultural Conservatism: A New National Agenda.

At Yale, Marshner became a prominent leader of campus conservatism, opposing the anti-Vietnam War movement and student radicalism of the late 1960s.

At Yale Graduate School, he was given a bursary job as night watchman in Sterling Memorial Library, where he read Thomas Aquinas.

The combined influences of Bozell, Wilhelmsen, and Aquinas provoked a religious crisis, causing Marshner to question Lutheranism and to convert to the Catholic Church.