Carl Otto Robert Peter Paul Graebner was a German botanist.
In 1895 he obtained his doctorate in Berlin, successively working as an assistant and then as curator at the botanical gardens. During the 1890s he performed botanical investigations in Jerichower Land and Vorharz with Paul Ascherson.
He later became a professor at the Botanical Garden and Museum in Berlin-Dahlem, from where he conducted floristic and phytogeographical research.