August Wilhelm Eichler, also known under his Latinized name, Augustus Guilielmus Eichler (April 22, 1839 – March 2, 1887), was a German botanist who modified the classification system to better reflect the relationships between plants. He divided the plant kingdom into non - floral plants (Cryptogamae) and floral plants (Phanerogamae).

The Eichler System was the first to accept the concept of evolution and therefore also the first to be considered phylogenetic. Moreover, Eichler was the first taxonomist to separate the Phanerogamae into Angiosperms and Gymnosperms and the former into Monocotyledonae and Dicotyledonae.

The Eichler system was the foundation for Adolf Engler's System and was widely accepted in Europe and other parts of the world.

Born in Neukirchen, Hesse, Eichler studied at the University of Marburg, Germany, and in 1871 became Professor of Botany at Technische Hochschule (Technical University), Graz. In 1872 he received an appointment at the University of Kiel, where he remained until 1878 when he became director of the herbarium at the University of Berlin. He died in Berlin on March 2, 1887.

Eichler made important contributions to the study of the comparative structure of flowers (mainly on floral symmetry in his work Blütendiagramme).