German theologian, lecturer, editor, and polemicist who is remembered chiefly for his long public controversy with Martin Luther (Luther, Martin) at the onset of the Reformation.

Emser studied humanities at the University of Tübingen and jurisprudence and theology at the University of Basel. In 1504 he lectured on classics at Erfurt (where Luther may have been among his listeners) and became secretary to Duke George of Saxony. He was ordained a priest circa 1512.

Emser first sided with the reformers, but he desired a practical reformation of the clergy without any doctrinal breach with the past or with Rome. His sympathies were mainly humanistic. The radical opinions expressed by Luther at the theological debate at Leipzig (1519) brought their relationship to an open break. In the ensuing bitter controversy (which lasted until his death), Emser wrote eight polemical tracts against Luther (1520–21). He also entered into a controversy with Huldrych Zwingli, the influential Swiss Reformer.

In 1527 Emser published a German translation of the New Testament, from the Vulgate, with annotations. It was meant to counter Luther's translation, but it remained essentially a revision of Luther's work. Nevertheless, by the end of the 18th century Emser's translation had gone through more than 100 editions.