An aecium or aecia is a type of spore-bearing structure occurring in the secondary fruiting stage of rust fungi (Pucciniales), which grow as endoparasites of vascular plants. It consists of a mycelium with dikaryotic cells that produces aeciospores by budding. Depending on the form of their aecia, rust fungi whose main fruiting stage is unknown are traditionally divided into form genera: Caeoma for simple aecia without peridia, Aecidium for cup-shaped spore-bearing structures with peridia open at the top, Peridermium for aecia with walls initially closed like a dome, and Roestelia for species producing lattice-like peridia.

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