Karl Waldemar Ziegler (German: [kaːʁl ˈvaldəˌmaʁ ˈt͡siːɡlɐ]; 26 November 1898 – 12 August 1973) was a German chemist who won the Nobel Prize in Chemistry in 1963, with Giulio Natta, for work on polymers. The Nobel Committee recognized his "excellent work on organometallic compounds [which]...led to new polymerization reactions and ... paved the way for new and highly useful industrial processes". He is also known for his work involving free-radicals, many-membered rings, and organometallic compounds, as well as the development of Ziegler–Natta catalyst. One of many awards Ziegler received was the Werner von Siemens Ring in 1960 jointly with Otto Bayer and Walter Reppe, for expanding the scientific knowledge of and the technical development of new synthetic materials.

Karl Ziegler was born on 26 November 1898 in Helsa near Kassel, Germany and was the second son of Karl Ziegler, a Lutheran minister, and Luise Rall Ziegler. He attended Kassel-Bettenhausen in elementary school. An introductory physics textbook first sparked Ziegler's interest in science. It drove him to perform experiments in his home and to read extensively beyond his high school curriculum. He was also introduced to many notable individuals through his father, including Emil Adolf von Behring, recognized for the diphtheria vaccine. His extra study and experimentation help explain why he received an award for most outstanding student in his final year at high school in Kassel, Germany. He studied at the University of Marburg and was able to omit his first two semesters of study due to his extensive background knowledge. His studies were interrupted however, as during 1918 he was deployed to the front as a soldier to serve in World War I. He received his Ph.D. in 1920, studying under Karl von Auwers. His dissertation was on "Studies on semibenzole and related compounds" which led to three publications.

Karl Ziegler showed an eagerness for science at an early age. He progressed through schooling quickly receiving a doctorate from the University of Marburg in 1920. Soon after, he briefly lectured at the University of Marburg and the University of Frankfurt.

In 1926 he became a professor at the University of Heidelberg where he spent the next ten years researching new advances in organic chemistry. He investigated the stability of radicals on trivalent carbons leading him to study organometallic compounds and their application in his research. He also worked on the syntheses of multi-membered ring systems. In 1933 Ziegler published his first major work on large ring systems, "Vielgliedrige Ringsysteme" which presented the fundamentals for the Ruggli-Ziegler dilution principle.

In 1936 he became Professor and Director of the Chemical Institute (Chemisches Institut) at the University of Halle-Saale and was also a visiting lecturer at the University of Chicago. Ziegler, who was a Patron Member of the SS received the War Merit Cross 2nd Class in October 1940.

From 1943 until 1969, Ziegler was the Director of the Max Planck Institute for Coal Research (Max-Planck-Institut fur Kohlenforschung) formerly known as the Kaiser-Wilhelm Institute for Coal Research (Kaiser-Wilhelm-Institut fur Kohlenforschung) in Mülheim an der Ruhr as a successor to Franz Fischer.

Karl Ziegler was credited with much of the postwar resurrection of chemical research in Germany and helped found the German Chemical Society (Gesellschaft Deutscher Chemiker) in 1949. He served as president for five years. He was also the president of the German Society for Petroleum Science and Coal Chemistry (Deutsche Gesellschaft für Mineralölwissenschaft und Kohlechemie), from 1954 to 1957. In 1971, The Royal Society, London, elected him as a Foreign Member.

In 1922, Ziegler married Maria Kurtz. They had two children, Erhart and Marianna. His daughter, Dr. Marianna Ziegler Witte was a doctor of medicine and married a chief physical of a children's hospital (at that time) in the Ruhr. His son, Dr. Erhart Ziegler, became a physicist and patent attorney. In addition to his children, Karl Ziegler has five grandchildren by his daughter, and five by his son. At least one of his grandchildren, Cordula Witte, attended his Nobel Prize reception as there is a picture of the two of them happily dancing. Ziegler enjoyed traveling around the world with his family, especially on cruises. He even charted special cruises and airplanes for eclipse viewing. It was during a 1972 eclipse-viewing cruise with his grandson that Karl Ziegler became ill. He died a year later.