Harvey Gallagher Cox Jr. (born May 19, 1929) is an American theologian who served as the Hollis Professor of Divinity at Harvard Divinity School, until his retirement in October 2009. Cox's research and teaching focus on theological developments in world Christianity, including liberation theology and the role of Christianity in Latin America.

Cox was born on May 19, 1929, in Phoenixville, Pennsylvania, to Dorothea Cox and Harvey Gallagher. He was raised in Malvern, Pennsylvania. After a stint in the US Merchant Marine, Cox attended the University of Pennsylvania and graduated in 1951 with a Bachelor of Arts degree with honors in history. He earned a Bachelor of Divinity degree from Yale Divinity School in 1955, and a Doctor of Philosophy degree in the history and philosophy of religion from Harvard University in 1963.

Cox was ordained as an American Baptist minister in 1957, and started teaching as an assistant professor at the Andover Newton Theological School in Massachusetts. He then began teaching at Harvard Divinity School (HDS) in 1965 and in 1969 became a full professor. He was to become "the single most heeded professor in religion at Harvard."

Cox became widely known with the publication of The Secular City in 1965. It became immensely popular and influential for a book on theology, selling over one million copies.^{[citation needed]} Cox developed the thesis that the church is primarily a people of faith and action, rather than an institution. He argued that "God is just as present in the secular as the religious realms of life". Thus, the original title was God and the Secular City, which "he still believes… would have more accurately described the book's theme."

Far from being a protective religious community, the church should be in the forefront of change in society, celebrating the new ways religiosity is finding expression in the world. Phrases such as "intrinsic conservatism prevents the denominational churches from leaving their palaces behind and stepping into God's permanent revolution in history" were viewed as threatening to the status quo by some, or seen as an embrace of the social revolution of the 1960s. Cox revisited his topic in Religion in the Secular City: Toward a Post-Modern Theology in 1984. In 1990, a twenty-fifth anniversary edition of The Secular City was published.

After his international best seller, Cox thought he might have "peaked" too soon at age 34, as he experienced a "second book crisis," but he then wrote The Feast of Fools (1969), which he has said "still remains my own favorite ... the 'one book' I recommend to people who ask me at parties which one of my books they should crack." Originally Cox presented it as the William Belden Noble Lecture at Harvard in 1968, which included music, dance, film, and balloons; and Cox himself was known to play the tenor saxophone in a jazz ensemble called The Embraceables. Celebrating his new book as Dionysian in playful dynamic with The Secular City as Apollonian, he said "there is an unnecessary gap in today's world between the world changers and the life-celebrators," thus promoting a "proleptic liberation as a festive radical."

In 1973, Cox wrote The Seduction of the Spirit, which he said "has the best first chapter of anything I have ever written (about my boyhood in Malvern, Pennsylvania, the churches there and my baptism)," but he added it went "downhill" from there.

In Turning East (1977), Cox describes his teaching at the Naropa Institute in Boulder, Colorado, where his mind and soul were challenged by the Buddhist "dharma", and he enjoyed doing research in Asian religious movements.