A deeper understanding of the phytochemical composition of perennial legumes is important for animal nutrition as well as for the development of pharmaceuticals. The present study is an assessment of perennial legumes including clovers (Trifolium pratense L. and T. medium L.), medics (Medicago sativa L. and M. lupulina L.), sainfoin (Onobrychis viciifolia Scop.) and milkvetches (Astragalus glycyphyllos L. and A. cicer L.) as potential sources of bioactive compounds. Whole aerial parts of plants at stem elongation and full-flowering stages as well as morphological parts of fully flowering plants (leaves, stems and flowers) were analysed for four isoflavones (biochanin A, daidzein, formononetin, and genistein), coumestrol, condensed tannins and triterpene saponins. According to total concentration of the four isoflavones in plants, the species were ranked as follows: T. medium (23.2–28.7 mg g–1 DM) > T. pratense > A. glycyphyllos = A. cicer = O. viciifolia = M. sativa = M. lupulina (0.054–0.212 mg g–1 DM). Young plants were 1.2–5.6-fold richer in isoflavones than fully flowering plants. Quantifiable concentrations of coumestrol and saponins were found in the Medicago species and tannins in O. viciifolia. In general, leaves contained the highest concentrations of the phytochemicals. These findings suggest considerable potential to improve legume-based forage quality through appropriate exploitation of species- or cultivar-specific phytochemicals and manipulation of the content of phytochemicals by management of production including growth stage at harvest and selection of plant species.