Karl Adolph Hansen was a German botanist. He was born in Hamburg and graduated in 1887 at the University of Würzburg with a thesis entitled "Geschichte der Assimilation und Chlorophyllfunktion". He was professor of botany at the Justus Liebig-Universität Gießen 1891-1920. He had very broad academic interests, including history and archaeology. However, he always worked alone, supervised very few doctoral students, and stood outside the development of experimental physiological botany among his contemporaries. Hansen wrote scholarly works on Goethe's Metamorphosis of Plants and had an aggressive dispute over this hypothesis with Houston Stewart Chamberlain. He died in Giessen.