Theromyzon is a genus of leeches belonging to the family Glossiphoniidae.

[1][2] Leeches of this genus parasitize birds and are sometimes called duck leeches, although their hosts are not limited to ducks.

The Theromyzon species tend to feed in the nasal cavities of waterbirds in general, from ducks to penguins.

As parasites of birds, and in many cases migratory waterfowl, these leeches typically have a broad distribution range.

Leeches of the Theromyzon genus have been observed in the Americas, Eurasia, and Africa.