Juan (Jean) Brèthes, also known as Frère Judulien Marie or Juan Brethes (24 February 1871, in Saint-Sever, Landes – 2 July 1928, in Buenos Aires) was an Argentine scientist, naturalist, entomologist, ornithologist, zoologist and geologist.
He was the first entomologist of the National Museum, today known as the Argentine Museum of Natural Sciences.
He was a close collaborator of Florentino Ameghino, and translated several of his works into French.
Thanks to his intense activity, he systematized a large number of Latin American insect species.
He was a precursor in the fight against agricultural pests at a time when insecticides had not been developed to combat them.