Ludwig Knorr (December 2, 1859 – June 4, 1921) was a German chemist. Together with Carl Paal, he discovered the Paal-Knorr synthesis, and the Knorr quinoline synthesis and Knorr pyrrole synthesis are also named after him. The synthesis in 1883 of the analgesic drug Antipyrin, now called Phenazone, was a commercial success. Antipyrin was the first synthetic drug and the most widely used drug until it was replaced by Aspirin in the early 20th century.