Monosiga is a genus of collared flagellates (choanoflagellates). The genus typifies one of the three main choanoflagellate families, the Codosigidae (the others are the Salpingoecidae and the Acanthoecidae). Species occur in marine and in freshwater habitats, as single cells attached to larger structures or organisms, and together with other protists can account for significant percentages of total consumption of bacteria and bacterial-sized eukaryotes in aquatic ecosystems.

Choanoflagellates closely resemble the choanocytes of sponges and other animals, especially invertebrates. For this reason, choanoflagellates have long been thought to be the protist group that is the most closely related to the animals; this hypothesis was first published in 1866. Molecular investigations are showing that choanoflagellates belong to a clade that includes the animals, the fungi, the ichthyosporean/mesomycetozoan group of commensal/parasitic protists, and the nucleariid amoebae.