Pierre François Marie Auguste Dejean (10 August 1780 – 17 March 1845), was a French entomologist.
A soldier of fortune during the Napoleonic Wars, he rose to the rank of lieutenant general and aide de campe to Napoleon.
He amassed vast collections of Coleoptera, some even collected on the battlefield at Waterloo.
He listed 22,399 species in his cabinets in 1837—at the time, the greatest collection of Coleoptera in the world.
In 1802, he began publishing a catalogue of his vast collection, including 22,000 species names.
Dejean was an opponent of the Principle of Priority in nomenclature.
"I have made it a rule always to preserve the name most generally used , and not the oldest one; because it seems to me that general usage should always be followed and that it is harmful to change what has already been established".
Dejean acted accordingly and often introduced in litteris names, given by himself to replace those already published by other authors.
They became invalid.
Dejean was president of the Société entomologique de France for the year 1840.
In 1834, he was elected a foreign member of the Royal Swedish Academy of Sciences.