Swedish entomologist.

A member of a wealthy aristocratic Swedish family that had originated in Brabant (modern Belgium), De Geer himself grew up in Holland but returned to Sweden in 1739. Because of his wealth and heritage (rather than because of any youthful accomplishments), he was elected to the Swedish Academy of Sciences at age 19. Thereafter he proved himself by becoming the foremost 18th-century Swedish scientist after Carolus Linnaeus.

De Geer made novel use of the microscope and carried biological observation to the point of originating a system of insect classification using mouthparts and wings as criteria. His Mémoires pour servir à l'histoire des insectes (7 vol., 1752–78) parallels R.-A.-F. de Réaumur (Réaumur, René-Antoine Ferchault de)'s earlier work of the same title (6 vol., 1734–42).