In botany, a haustorium (plural haustoria) is the appendage or portion of a parasitic fungus (the hyphal tip) or of the root of a parasitic plant (such as the broomrape family or mistletoe) that penetrates the host's tissue and draws nutrients from it. Haustoria do not penetrate the host's cell membranes.

Fungi in all major divisions form haustoria. Haustoria take several forms. Generally, on penetration, the fungus increases the surface area in contact with host plasma membrane releasing enzymes that break down the cell wall, enabling greater potential movement of organic carbon from host to fungus. Thus, an insect hosting a parasitic fungus such as Cordyceps may look as though it is being "eaten from the inside out" as the haustoria expand inside of it.

Haustoria arise from intercellular hyphae, appressoria, or external hyphae. The hypha narrows as it passes through the wall of the cell and then expands on invaginating the cell. A thickened, electron-dense collar of material is deposited around the hypha at the point of invagination. Further, the host wall becomes highly modified in the invaginated zone. Inclusions normally present in plasma membrane are absent, and the outer layer contains more polysaccharide. The wall of both partners is severely reduced.

Functional exchange takes place within the haustorial complex. The host supplies organic carbon to the fungus, and the metabolic activity within the complex is considerably greater than outside. Carbon from the host is absorbed by the fungus, and immediately transported to the rest of the thallus. The host plant appears to be functioning according to signals from the fungus and the complex appears to be under the control of the invader.

The haustorium may be balloon-, spiral- or glove-shaped.