Year 521 (DXXI) was a common year starting on Friday of the Julian calendar. At the time, it was known as the Year of the Consulship of Sabbatius and Valerius (or, less frequently, year 1274 Ab urbe condita). The denomination 521 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

• Future Byzantine emperor Justinian, age 39, is appointed consul. He later becomes Commander-in-chief of the Army of the East.

• Ma`adikarib Ya`fur becomes king, supported by the Aksumites; he begins a military campaign against the Arabian tribes.

• Boethius introduces Greek musical letter notation to the West.

• February 22 – Samson of Dol is ordained as bishop in Brittany, on the Feast of the Chair of Saint Peter.
• Ecclesius becomes a bishop of Ravenna.^[1]

• December 7 – Columba, Gaelic Irish missionary monk (d. 597)
• Agericus, bishop of Verdun (approximate date)
• Gao Cheng, high official and regent of Eastern Wei (d. 549)

• July 17 – Magnus Felix Ennodius, bishop and Latin poet
• November 29 – Jacob of Serugh, Syrian poet and theologian