Karl Theodor Jaspers was a German-Swiss psychiatrist and philosopher who had a strong influence on modern theology, psychiatry, and philosophy. After being trained in and practicing psychiatry, Jaspers turned to philosophical inquiry and attempted to discover an innovative philosophical system. He was often viewed as a major exponent of existentialism in Germany, though he did not accept the label.

Karl Johannes Neumann was a German classical historian.

Karl Josef von Hefele was a Roman Catholic bishop and theologian of Germany.

Karl Joseph Simrock was a German poet and writer. He is primarily known for his translation of Das Nibelungenlied into modern German.

Karl Julius Beloch was a German classical and economic historian.

Karl Julius Weber (16 or 20 April 1767, in Langenburg – 19 July 1832, in Kupferzell; also written Carl Julius Weber) was a German short story writer.

Karl Johann Kautsky was a Czech-Austrian philosopher, journalist, and Marxist theorist. A leading theorist of the Social Democratic Party of Germany (SPD) and the Second International, Kautsky advocated orthodox Marxism, which emphasized the scientific, materialist, and determinist character of Karl Marx's work. This interpretation dominated European Marxism for two decades, from the death of Friedrich Engels in 1895 to the outbreak of World War I in 1914.

Karl Gustav Adolf Knies was a German economist of the historical school of economics, best known as the author of Political Economy from the Standpoint of the Historical Method (1853). Knies taught at the University of Heidelberg for over 30 years, and was perhaps the most theoretically oriented economist of the older historical school.

Karl Knortz was a German-American author.

Karl Korsch was a German Marxist theoretician and political philosopher. He is recognized as one of the "dissidents" that challenged the Marxism of the Second International of Karl Kautsky, Georgi Plekhanov and Lenin. Along with György Lukács, Korsch is considered to be one of the major figures responsible for laying the groundwork for Western Marxism in the 1920s.