Ludwig Kaas (23 May 1881 – 15 April 1952) was a German Roman Catholic priest and politician during the Weimar Republic.
Born in Trier, Kaas was ordained a priest in 1906 and studied history and Canon law in Trier and Rome. 1906 he completed a doctorate in theology and in 1909 he obtained a second doctorate in philosophy. In 1910 he was appointed rector of an orphanage and boarding school near Koblenz. Until 1933, he devoted his spare time to scholarly pursuits. In 1916 he published the book "Ecclesiastical jurisdiction in the Catholic Church in Prussia" (Die geistliche Gerichtsbarkeit der katholischen Kirche in Preußen in Vergangenheit und Gegenwart mit besonderer Berücksichtigung des Wesens der Monarchie), demonstrating his expertise in church history, Canon law and his political interests. In 1918 he requested to be sent to a parish, but Michael Felix Korum of Trier refused and instead appointed him professor of canon law at the Trier seminary in 1918. In that position, he published the study "Missing in war and remarriage in state law and canon law" (Kriegsverschollenheit und Wiederverheiratung nach staatlichen und kirchlichen Recht), dealing with remarriage in case of spouses missing in war. In 1919 he was offered the chair for canon law at the university of Bonn and was initially inclined to accept it, but as he did not find the conditions in Bonn to his liking and after consultation with Bishop Korum he refused the offer.
Distressed by the revolution, Kaas also decided to engage in politics and joined the Centre Party. In 1919 he was elected to the Weimar National Assembly and in 1920 to the Reichstag, of which he was a member until 1933. He was also elected to the Prussian state council, the representation of Prussia's provinces. As a parliamentarian Kaas specialized in foreign policy. From 1926 to 1930 he was a German delegate to the League of Nations.