Joseph Gottlieb Kölreuter (27 April 1733 – 11 November 1806), also spelled Koelreuter or Kohlreuter, was a German botanist who pioneered the study of plant fertilization, hybridization and was the first to detect self-incompatibility.

At the age of fifteen he went to study medicine at the University of Tübingen under physician and botanist Johann Georg Gmelin who had returned from St. Petersburg.

Gmelin had an interest in floral biology and he reprinted a work by Rudolf Jakob Camerarius (who also taught at Tübingen) who was the first to demonstrate sexual reproduction in plants.

Gmelin died in 1755, and Kolreuter earned his degree and received an appointment at the Imperial Academy of Sciences at St. Petersburg on 23 December 1755.

He moved to Calw in 1763 and Karlsruhe in 1764 where he was briefly professor of natural history and director of the botanical garden at Baden.

Linnaeus through his student J. J. Hartmann reported the possibility of new "species" arising from hybridization but Kölreuter was skeptical of the results.

Kölreuter collected nectar from many hundreds of orange trees and kept them in vials to evaporate and he reported that it thickened and tasted like honey with time.

[1] Kölreuter followed an idea from alchemistry that metals were a mixture of mercury and sulphur and considered likewise that an equilibrium of the male and female "seed matters" had a role in deciding the qualities of hybrid offspring.

[2] Although Koelreuter did not endorse the transmutation of species, his hybridisation research influenced the development of evolutionary theory in the eighteenth century.