Nancy J. Duff (born May 2, 1951) is an American professor of theology. Duff worked as the Stephen Colwell Associate Professor of Christian Ethics at Princeton Theological Seminary, where she taught from 1990 until 2020. Duff is also a Presbyterian minister in the PCUSA denomination. She is married to United Methodist Minister David Mertz. She has taught courses on the Decalogue, Biomedical ethics, human sexuality, liturgy and the Christian life, Dietrich Bonhoeffer, James Cone, types of Christian ethics, and vocation in Christian tradition and contemporary life.