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]