Hydromermis osami n. sp. (Nematoda: Mermithidae) is described from the hemocoel of 2 species of Rheotanytarsus chironomid imagos eclosing from Lake Ozawindib and Gulsvig Landing in northern Minnesota during August 2002 and June and July 2006. The species is distinguished from the other 26 described members of the genus by the terminal mouth; oval, opaque, thick walled amphids, wider than long; acute posterior end; single spicule not bifurcated proximally; well-developed uterine and vulval limbs of the S-shaped vagina; absence of bursal sleeve; absence of nutrient vesicles in the trophosome; and an esophagus length over 40% of body length. Members of the new species emerge from the hosts as adults. The other described Hydromermis species, number of specimens, location, and known hosts are tabulated.