Taxa belonging to the genus Miscanthus L. (silvergrass) are non-invasive plants that can grow on marginal land that are not suitable for food production. These species give high yields of lignocellulosic biomass, being a rich and renewable source of organic matter for the production of second-generation biofuels (ethanol) and for other practical purposes, such as raw material for paper production. The most promising species of the genus Miscanthus L., in terms of practical use in industry and agriculture, are M. sinensis, M. sacchariflorum and their hydride - M. × giganteus. In this study, extracts obtained from the green biomass of these Miscanthus L. species underwent detailed phytochemical investigation and multidirectional biological evaluation. The extracts were prepared using supercritical fluid extraction with water as co-solvent. The qualitative RP-HPLC/PDA analysis of the extracts after their purification with solid-phase extraction confirmed their similar phytochemical profiles. Six free phenolic acids (p-hydroxybenzoic, isovanillic, syringic, p-coumaric, ferulic and isoferulic) and one trans-cinnamic acid derivative were identified and determined. The antimicrobial, antiviral, anthelmintic and anticancer activity of the supercritical extracts of Miscanthus L. species studied was revealed for the first time. Moreover, the extracts possessed low cytotoxicity against normal cells. This screening evaluation showed that, besides giving high yields of lignocellulosic biomass, these grasses could also be a source of valuable bioactive compounds for the pharmaceutical and cosmetic industries.