A hypha is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium

In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium.

A hypha consists of one or more cells surrounded by a tubular cell wall. In most fungi, hyphae are divided into cells by internal cross-walls called "septa" (singular septum). Septa are usually perforated by pores large enough for ribosomes, mitochondria, and sometimes nuclei to flow between cells. The major structural polymer in fungal cell walls is typically chitin, in contrast to plants and oomycetes that have cellulosic cell walls. Some fungi have aseptate hyphae, meaning their hyphae are not partitioned by septa.

Hyphae have an average diameter of 4–6 μm.