or Herpetomonas, a genus of protozoans of the order Protomonadina, class Mastigophora, with an elongate body (15–30 microns), a single nucleus, and a single flagellum with a blepharoblast at the base.

There are more than ten species of Leptomonas. All are parasites of animals or plants. They infest the intestines of insects and reptiles and the blood of fish, birds, and mammals. They parasitize in the latex of some plants (Euphorbiaceae, Compositae); they severely injure and often cause the death of coffee plants by inducing necrosis of the phloem. The carriers of the species that infest plants are bugs.