The pycnidia - Singular Pyknidie or Pycnidium - are spherical to flask-shaped fruit body-like structures (also known as fructifications ) that occur in the group of the Ascomycota. In its interior are very small spores, the conidia are formed. Asexual spores that arise in pycnidia are often also called pycnospores.

Pycnidia are often found in lichens (ie lichen sac fungi ), where they are usually sunk into the camp of the lichen and often, their tiny, superficial opening, the ostiole visible.

In non- lichen Ascomycetes the pycnidia are more or less depressed in the host tissue. This Ascomycota form the shape of the taxon order Sphaeropsidales. This includes important pathogens of plant diseases such as Ascochyta chrysanthemi or Phoma lingam.

The rust fungi corresponding structures are also called spermogonium.