Hydrachna is a genus of mites in the family Hydrachnidae, the sole genus of the family. There are more than 80 described species in Hydrachna. Larvae of this genus are known to be parasites of water beetles such as Eretes griseus by attaching to the back of the thorax and abdomen. These water mite larvae are also parasites of Callicorixa, Corixa, Cymatia and Sigara species, although Sigara falleni has been described as "immune" to these mites, and Cymatia coleoptrata and Sigara striata also gain this "immunity" after a few years of interaction.

These 87 species belong to the genus Hydrachna: