Abstract Various parts of guelder rose ( Viburnum opulus L.), including the bark, leaves, flowers, and fruits, have long been used in modern and folk medicine. This study aims to evaluate how site climatic conditions and soil properties influence the accumulation of phenolic antioxidants in flowers and fruits of Viburnum opulus L. It was established that the content of total phenolics in fruits was 1.7 times that in the flowers; the content of tannins, 2.5 times; antioxidant activity, 2.3 times. A high correlation between antioxidant capacity and the content of phenolic compounds (r = 0.90; p <0.001) and tannins (r = 0.94; p <0.001) was revealed. Site conditions have a strong effect both on the accumulation of secondary metabolites in Viburnum opulus L. and on the antioxidant activity of the study sample extracts. The highest concentrations of the phytocomponents under study were associated with heavy precipitation and high average contents of organic matter and macroelements (phosphorus, potassium, and nitrogen). These findings highlight the importance of fulfilling agro-technical requirements for the cultivation of Viburnum opulus L., both to ensure high yields and to obtain products with better biochemical composition.