Plenasium is a genus of ferns in the family Osmundaceae.

[1] It is recognized in the Pteridophyte Phylogeny Group classification of 2016 (PPG I),[2] but kept within a more broadly circumscribed genus Osmunda by other sources.

[3] The genus is known from Early Cretaceous to present.

[4] As of October 2019[update], the Checklist of Ferns and Lycophytes of the World accepted the following four species:[1] Phylogeny of Plenasium[5][6] Plenasium angustifolium (Ching) A.E.Bobrov Plenasium vachellii (Hook.)

C.Presl Plenasium banksiifolium (C.Presl) C.Presl Plenasium javanicum (Blume) C.Presl The following fossil species are also accepted.