沼兰（学名：Malaxis monophyllos）为兰科沼兰属下的一个种。

Malaxis monophyllos, the white adder's mouth, is a terrestrial species of orchid. It is widespread across much of Europe (Germany, Italy, Poland, Scandinavia, Ukraine, etc.), Asia (China, Japan, Russia, Nepal, Philippines, etc), and much of southern Canada. In the United States, it grows mostly in southern Alaska, New England and the Great Lakes region, with isolated populations reported from Colorado and California.