An archegonium (pl: archegonia) (from the Greek arche = beginning and gonos = born) is a multicellular structure or organ of the gametophyte phase of certain plants producing and containing the ovum or female gamete. Archegonia are typically located on the surface of the plant thallus, although they are embedded in the horned liverworts. They are much reduced and embedded in the megasporangium of gymnosperms. The term is not used for angiosperms or the gnetophytes, Gnetum and Welwitschia, because the comparable "structure" is reduced to just a few cells, and the function of surrounding the gamete completely assumed by diploid cells of the megasporangium.

The corresponding male organ is called the antheridium.