Dwarf birch (Betula nana L.), a dominant deciduous dwarf shrub in many tundra ecosystems, is predicted to increase substantially in abundance due to climate warming. Potential warming-induced changes in the concentrations of phenolic compounds in B. nana leaves could influence the susceptibility of B. nana to environmental stresses; however, only a few studies have investigated the effects of climate warming on the phenolic defence in B. nana. We analysed the responses of phenolic concentrations and amounts in B. nana leaves to factorial treatments of warming and fertilization for 2 years in a subarctic tundra heath. Warming induced a strong decrease in total phenolics, including both flavonols (i.e. quercetin and myricetin derivatives, important defence compounds against oxidative stress) and hydrolysable tannins (HTs, important defence compounds against herbivory). Fertilization exerted weaker effects on phenolic concentrations while significantly increasing the leaf area. Our data indicate that climate warming may decrease the level of phenolic defence in B. nana leaves. Given the important role of these compounds in the plant defence, this phenomenon could potentially increase the susceptibility of B. nana to biotic stresses such as herbivory.