卵菌门（学名：Oomycota）或卵菌纲（学名：Oomycetes），俗称水霉，是一种与真菌很相似的真核微生物，但根据亲缘关系学，两者的来源并不相同。本物种的生物呈丝状、微细且会吸水，能够进行有性繁殖和无性繁殖。卵菌纲有腐生和寄生两类生活方式，均为嗜水的寄生性，是不少植物性瘟疫的元凶，如爱尔兰马铃薯饥荒及橡树突然死亡，都是由卵菌纲生物引起的。

卵菌纲生物的菌丝在孢子囊的底部，之间很少有间隔，即使有也很罕见，有些是单细胞，但其他的有丝状分支。

Oomycota or oomycetes (/ˌəʊəˈmaɪsiːt/) form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms. They are filamentous, microscopic, absorptive organisms that reproduce both sexually and ****ually. Oomycetes occupy both saprophytic and pathogenic lifestyles, and include some of the most notorious pathogens of plants, causing devastating diseases such as late blight of potato and sudden oak death. One oomycete, the mycoparasite Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi. The oomycetes are also often referred to as water molds (or water moulds), although the water-preferring nature which led to that name is not true of most species, which are terrestrial pathogens. The Oomycota have a very sparse fossil record. A possible oomycete has been described from Cretaceous amber.