Baron Charles de Geer (the family is usually known as De Geer with a capitalized "De"; Finspång in Risinge 30 January 1720 – Stockholm 7 March 1778) was a Swedish industrialist and entomologist.

Life
De Geer was a great admirer of Réaumur. Hence his modelling Mémoires pour servir à l'histoire des insectes on Réaumur's work of the same title. It, too, is in French, similarly in large quarto and with the same decorations. The Mémoires deal with 1,466 species, treating life histories, food and reproduction based on careful, patient investigation and analysis of existing literature. There are 238 copper plates. The descriptions are acutely observed.
In nomenclature de Geer was less progressive; Volume 1 of the Mémoires (1752) was too early to employ the binomial system invented by his fellow Swede Linnaeus. Volume 2 (1771) does not use it, and in Volume 3 (1773) the system is only partially employed. Here the specific name is placed in square brackets and is followed by a long diagnosis in the older style. He also changed many of Linnaeus' names. It seems that this was a concession to usage as in the 1760s and 1770s the Linnean system became increasingly employed, not because de Geer liked the new system. They had differences "not everyone sees things in the same light, and people have the weakness of frequently being too fond of their own opinions" (letter to Linnaeus 16 October, 1772) and "if here and there I am stll of a different opinion, I am now, as before, asking you not to take it amiss" (letter to Linnaeus 23 February 1774).