The diplomonads are a group of flagellates, most of which are parasitic. They include most notably Giardia lamblia, which causes giardiasis in humans. They are placed among the eukaryota (they are one of the rare groups of eukaryota that do not contain organelles), metamonads, and appear particularly close relatives of the retortamonads .

With a few exceptions, diplomonads are double cells: they have two nuclei, each with four associated flagella, arranged symmetrically about the body's main axis. Like their relatives, they lack mitochondria, but are now known to possess mitochondrial relics called mitosomes . These are not used in ATP synthesis the way mitochondria are, but are involved in the maturation of iron-sulfur proteins.