Euthymios Zigabenos (Greek: Εὐθύμιος Ζιγαβηνός; died after 1118) was a 12th-century Byzantine Greek monk and theologian known for his prolific writings and staunch defense of Orthodox Christianity.

Details about Zigabenos' life are scarce. It is known that he was a monk residing in a monastery dedicated to the Virgin Mary near Constantinople. He gained the favor of Emperor Alexios I Komnenos and his daughter, Anna Komnene, who praised his erudition and piety in her historical work, the Alexiad.

Zigabenos is best known for his theological treatises, notably the Panoplia Dogmatike ("Full Armor of Doctrine"). Commissioned by Emperor Alexios I, this extensive work systematically refutes various heresies, ranging from early Christian sects to contemporary movements like the Bogomils. The Panoplia Dogmatike remains a valuable source of information on these groups, particularly the Bogomils, whom Zigabenos personally encountered.

In addition to his polemical writings, Zigabenos authored commentaries on the Psalms, the Gospels, and the Pauline Epistles. These commentaries, largely based on the writings of the Church Fathers (patristic sources), demonstrate his deep knowledge of Scripture and tradition.

Euthymios Zigabenos' writings played a crucial role in upholding Orthodox Christianity during a time of intellectual and spiritual ferment. His works, particularly the Panoplia Dogmatike, continue to be studied by theologians and historians interested in Byzantine religious thought and the history of Christian heresies.