Kaspar Ulenberg (24 December 1549 – 16 February 1617) was a Catholic convert, theological writer and translator of the Bible.
He was born at Lippstadt on the Lippe, Westphalia, in 1549; died at Cologne, 16 February 1617. He was the son of Lutheran parents and was intended for the Lutheran ministry. He received his grammar-school education in Lippstadt, Soest, and Brunswick, and from 1569 studied theology at Wittenberg. While studying Luther's writings there his first doubts as to the truth of the Lutheran doctrines were awakened, and were then increased by hearing the disputes between the Protestant theologians and by the appearance of Calvinism in Saxony. After completing his studies he taught for a short time in the Latin school at Lunden in Dithmarschen; he was then sent by his family to Cologne to convert to Protestantism a kinsman who had become Catholic. After accomplishing this task he remained in Cologne, where, through his friendship with Johann Nopelius and Gerwin Calenius (Catholic countrymen of his), he had an opportunity of becoming acquainted with Catholic life and teaching.

In 1572 he became a Catholic, and soon afterwards, upon obtaining degrees in philosophy at the University of Cologne, became professor at the Gymnasium Laurentianum at Cologne. In 1575 he was ordained priest and became parish priest at Kaiserswerth. In 1583 he was made parish priest of St. Cunibert's in Cologne, where he laboured zealously by preaching and catechetical exercises, and made many conversions. In 1593 he became regent of the Laurentian gymnasium, retaining this position for twenty-two years. From 1600 to 1606 he directed the education of princes Wilhelm and Hermann of Baden, sons of Margrave Edward Fortunatus of Baden-Baden. In 1605 he became parish priest of St. Kolumba, Cologne, and from 1610 to 1612 was also rector of the university there.