水螅纲（Class Hydrozoa），又名水螅水母纲（学名Hydroidomedusa），是刺胞动物门水螅虫总纲的一个纲，包括了所有的水母及水螅类动物。

水螅纲动物的分类如下：

Hydrozoa (hydrozoans, from ancient Greek ὕδρα, hydra, "sea serpent" and ζῷον, zoon, "animal") are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most living in salt water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in fresh water. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.