Thomas of Celano (c. 1200 – c. 1265) was a 13th-century Italian Franciscan friar, chronicler, poet, and early companion of St. Francis of Assisi. Born in Celano, a small town near Lake Fucino in the Abruzzo region, he joined the newly established Franciscan Order around 1214 during the lifetime of St. Francis. In 1221, he was among the first group of friars sent on a mission to Germany, where he served as guardian of convents in Worms, Mainz, and Cologne, and eventually as sole custodian of the Rhine districts until his return to Assisi in 1230.

Following the canonization of St. Francis, Pope Gregory IX commissioned Thomas to record the official history of the saint's life. He authored the First Legend (Vita Prima, 1229), the Second Legend (Vita Secunda, 1247), and the Tract on the Miracles of St. Francis (Tractatus de Miraculis), which remain the primary biographical sources for early Franciscan history. He also composed the official Legend of St. Clare (Vita Sanctae Clarae) in 1255.

In the realm of hymnody, Thomas of Celano is traditionally regarded as the author of the monumental 13th-century Latin sequence "Dies Irae" ("Day of Wrath"). Composed in rhymed trochaic dimeter, this sequence offers a dramatic and solemn meditation on the Last Judgment, the heavenly trumpet, and the soul's plea for divine mercy at the throne of Christ. Adopted into the Roman Missal as the sequence for the Mass for the Dead (Requiem Mass), "Dies Irae" became one of the most famous and widely translated Latin hymns in Western church music, inspiring musical settings by composers across centuries and spawning dozens of English metrical versions.

His major hymnological contributions and translations include:

• "Dies Irae, dies illa" — The landmark Latin sequence on the Last Judgment and divine redemption.

• "Day of wrath! O day of mourning!" — The widely adopted English translation by William Josiah Irons (1848).

• "That day of wrath, that dreadful day" — Sir Walter Scott's famous poetic paraphrase from The Lay of the Last Minstrel (1805).

• "Day of anger, day of wonder" and "Day of wrath, that day of burning" — Popular Protestant and Anglo-Catholic adaptations used across 19th and 20th-century hymnals.

Thomas of Celano spent his final years as spiritual director to the Clarisses at the convent of San Giovanni Val dei Varri, where he died around 1265. His biographical writings and attribution to "Dies Irae" secure his position as a central literary figure of medieval Franciscan devotion.