Desiderius Erasmus Roterodamus (/ˌdɛzɪˈdɪəriəs ɪˈræzməs/ DEZ-i-DEER-ee-əs irr-AZ-məs; Dutch: [ˌdeːziˈdeːrijʏs eːˈrɑsmʏs]; 28 October c. 1466 – 12 July 1536), commonly known in English as Erasmus of Rotterdam or simply Erasmus, was a Dutch humanist, Christian theologian, and philosopher. He was, through his writings, one of the most influential scholars of the Northern Renaissance and a major figure of Western culture.

Erasmus was an important figure in classical scholarship who wrote in a spontaneous, copious and natural Latin style. As a Catholic priest developing humanist techniques for working on texts, he prepared pioneering new Latin and Greek scholarly editions of the New Testament and of the Church Fathers, with annotations and commentary that were immediately and vitally influential in both the Protestant Reformation and the Catholic Reformation. He also wrote On Free Will, The Praise of Folly, The Complaint of Peace, Handbook of a Christian Knight, On Civility in Children, Copia: Foundations of the Abundant Style and many other popular and pedagogical works.

Erasmus lived against the backdrop of the growing European religious reformations. He developed a biblical humanistic theology in which he advocated the religious and civil necessity both of peaceable concord and of pastoral tolerance on matters of indifference. He remained a member of the Catholic Church all his life, remaining committed to reforming the church from within. He promoted what he understood as the traditional doctrine of synergism, which some prominent reformers such as Martin Luther and John Calvin rejected in favour of the doctrine of monergism. His influential middle-road approach disappointed, and even angered, partisans in both camps.

Erasmus was the most popular, most printed and arguably most influential author of the early sixteenth century, read in all nations in the West and frequently translated. By the 1530s, his writings accounted for 10–20% of book sales in Europe. "Undoubtedly he was the most read author of his age." His vast number of Latin and Greek publications included translations, paraphrases, letters, textbooks, plays for schoolboys, commentary, poems, liturgies, satires, sermons, and prayers. A large number of his later works were defences of his earlier work from attacks by Catholic and Protestant theological and literary opponents.

The Catalogue of the Works of Erasmus (2023) runs to 444 entries (120 pages), almost all from the latter half of his life. He usually wrote books in particular classical literary genres with their different rhetorical conventions: complaint, diatribe, dialogue, encomium, epistle, commentary, liturgy, sermon, etc. His letter to Ulrich von Hutten on Thomas More's household has been called "the first real biography in the real modern sense."

From his youth, Erasmus had been a voracious writer. Erasmus wrote or answered up to 40 letters per day, usually waking early in the morning and writing them in his own hand. He did not work after dinner. His writing method (recommended in De copia and De ratione studii) was to make notes on whatever he was reading, categorised by theme: he carted these commonplaces in boxes that accompanied him. When assembling a new book, he would go through the topics and cross out commonplace notes as he used them. This catalogue of research notes allowed him to rapidly create books, though woven from the same topics. Towards the end of his life, as he lost dexterity, he employed secretaries or amanuenses who performed the assembly or transcription, re-wrote his writing, and in his last decade, recorded his dictation; letters were usually in his own hand, unless formal. For much of his career he wrote standing at a desk, as shown in Dürer's portrait.

In the second half of his life, Erasmus worked on New Testament studies in a project that eventually saw his intended detailed Annotations on the New Testament (modeled on a work by Lorenzo Valla) expand with a revised Vulgate recension, his own new Latin translation, an accompanying Greek text, essays justifying his approach, and lengthy Paraphrases of the entire New Testament except Revelation.

These went through multiple revisions and editions, and progressively involved many leading scholars and introduced several readings which were taken up by Protestant and Catholic reformers. Other publishers, such as Erasmus's earlier collaborator Aldine Press in Venice, immediately brought out their own editions, sometimes with their own corrections, and sometimes without the Annotations, or the Latin, or the Greek. Up to 300,000 copies of the various editions appear to have been printed in Erasmus's lifetime.