A hypha (plural hyphae) is a long, branching filament that collectively form the feeding structure of a fungus called the mycelium. Hyphae are also found enveloping the gonidia of lichens, making up a large part of their structure.

A typical hypha consists of tubular walls, usually made of chitin, which surround, support and protect the cellss that compose the hypha. For most fungi, a cell within a hypha filament is separated from other cells by internal cross-walls called septa (singular septum).

Some forms of parasitic fungi have a portion of their hyphae modified to form haustoria which are able to penetrate the tissues of a host organism. Similar, yet mutualistic forms of penetrating hyphae are called mycorrhizae and are very important in assisting nutrient and water absorption by plants.