One of the richest natural sources of resveratrol - the rhizome of Reynoutria japonica in East Asia is a well-known traditional herb (Hu zhang) used in various inflammatory diseases, infections, skin diseases, scald, hyperlipidemia. Although, it has been recently included in the European Pharmacopoeia, still in Europe is an untapped resource. Some of the therapeutic effects are likely to be influenced by its antioxidant properties and this in turn is frequently associated with a high stilbene content. However, some literatures suggested that other compounds than stilbenes may add to the total antioxidant capacity. Hence, the aim of this research was to examine rhizomes of R. japonica and less studied, morphologically similar species, R. sachalinensis and R. x bohemica for their phytochemical composition and antioxidant activity and to clarify the relationship between the antioxidant activity and compounds by statistical methods. HPLC/UV/ESI-MS studies of three Reynoutria species revealed 171 compounds comprising stilbenes, carbohydrates, procyanidins, flavan-3-ols, anthraquinones, phenylpropanoids, lignin oligomers, hydroxycinnamic acids, naphthalenes and their derivatives. Our studies confirmed the presence of procyanidins with high degree of polymerization, up to decamers in the rhizomes of R. japonica and brings new data on the presence of these compounds in other Reynoutria species. A procyanidin trimer digallate was described for the first time in the studied plants. Moreover, we suggested a presence of new for these species, dianthrone glycosides and previously unrecorded phenylpropanoid disaccharide esters and hydroxycinnamic acid derivatives, mainly in R. sachalinensis. Furthemore, compounds tentatively annotated as lignin oligomers were observed for the first time in studied species. The rhizomes of all Reynoutria species exhibited strong antioxidant activity. Statistical analysis demonstrated that proanthocyanidins should be considered as important contributors to the total antioxidant capacity.