John Behr FBA (born 16 October 1966) is a British Eastern Orthodox priest and theologian. Since 2020, he has served as the Regius Professor of Humanity at the University of Aberdeen. He is the former dean of St. Vladimir's Orthodox Theological Seminary, where he was the director of the Master of Theology Program and the Father Georges Florovsky Distinguished Professor of Patristics. He was ordained to the diaconate on 8 September 2001 and the priesthood on 14 September 2001. He served as the editor of the Popular Patristics Series, published by St. Vladimir's Seminary Press, from 1999 until 2020. He was elected dean of the seminary on 18 November 2006 and served from 2007 until 2017 when he was named Father Georges Florovsky Distinguished Professor of Patristics.

Behr completed his undergraduate studies at Thames Polytechnic, London, in 1987. His Bachelor of Arts dissertation was entitled "Of God, Man, and Creation: A Comparative Study of the Anthropology of the Greek Fathers and Emmanuel Levinas". Having studied under Bishop Kallistos Ware, Behr earned his Master of Philosophy degree in 1991 and Doctor of Philosophy degree in theology from Oxford University in 1995. Remnants of his master's thesis, "Sexuality, Marriage, and Asceticism in Second-Century Christian Writings", are found in his doctoral dissertation published in 2000 by Oxford University Press under the title Asceticism and Anthropology in Irenaeus and Clement. Behr completed a Master of Theology degree from St Vladimir's in 1997. His thesis, a complete textual criticism of On the Apostolic Preaching by Irenaeus of Lyons, was subsequently published by St. Vladimir's Seminary Press.

Behr has been Distinguished Lecturer at the Fordham University Theology Faculty, Visiting Professor of Historical Theology at the Nashotah House Theological Seminary, adjunct lecturer and faculty member of the St. Athanasius College which specialises in Coptic Orthodox theological studies. In September 2019, he was also appointed as professor in divinity at the Aberdeen University's School of Divinity, History and Philosophy.

He was elected a fellow of the British Academy in 2025.

Behr is married to a teacher of English and is the father of two sons and a daughter.

Behr's first major work, The Way to Nicaea, published in 2001, follows early Christian reflection beginning with the Scriptural Christ and continuing through to the Council of Antioch. This first of three volumes primarily consist of the examination of certain theologians: Ignatius, Justin Martyr, Irenaeus of Lyons, Hippolytus of Rome and the Roman Debates, Origen of Alexandria, and Paul of Samosata and the Council of Antioch. In his second volume, Behr provides a short biographical sketch of each, and then focuses on their works and the controversies they were engaged in. Neither a comprehensive history of theology nor a compendium of Christian doctrine, Behr instead draws attention to the theological debates and reflections that led up to the First Council of Nicaea.

Because the answer to a question can only be provided by its meaning, Behr begins with the question Christ himself asks, "Who do you say I am?" This answer, while provided by the Gospel, requires reflection—interpretation and explanation of this very person of Jesus Christ, his life and works. The writings of the New Testament, written from an interpretative confession of the crucified and risen Christ contemplated through Scripture, were the subject of intense debate and formation, eventually finding their normative foundation by the end of the 2nd century. According to Behr, this background is not only necessary for understanding later theological debates, but it is crucial to understanding those boundaries that identify Jesus Christ. It is the unique Jesus Christ-crucified on a cross, buried, risen three days later-contemplated through the texture of Scripture-the Law, the Psalms, and the Prophets-who is revealed as the Son of God. The incarnation, then, is not the single moment in which something began but rather a recapitulation—the same presence of the same word. It is through the Word of God previously hidden in the Scripture, as preached by the Apostles, revealed by the Holy Spirit, that the invisible, incomprehensible Father is made visible and comprehensible by the crucified and risen Jesus Christ. It is he who is always the Coming One, whom through continual contemplation and immersion in Scripture, devotion and death in his name, allows us to participate in the fullness of God.

Released in 2004, The Nicene Faith examines the theological reflection of the 4th century, beginning with the church shaking debates that led to Council of Nicaea and ending with their resolution at the Council of Constantinople. Like its predecessor, The Nicene Faith is structured not only chronologically, but according theme, examining of only certain theologians-Athanasius, Basil of Caesarea, Gregory of Nazianzus, and Gregory of Nyssa. Adapting a similar approach to The Way to Nicaea, this volume is neither a complete collection of theological discourses nor does it have a sole focus, such as Trinitarian theology. Rather, The Nicene Faith traces the development of theological reflection in the 4th century—Athanasius and the Cappadocians expounding their vision, preparing the path for the councils of Nicaea and Constantinople, providing the proper context in which their creeds can be correctly understood. One simply cannot reduce the results of these debates into shorthand formulae.