SummarySalicylic acid (SA) and jasmonic acid (JA) often play distinct roles in plant defence against pathogens. Research fromArabidopsis thalianahas established that SA‐ and JA‐mediated defences are more effective against biotrophs and necrotrophs, respectively. These two hormones often interact antagonistically in response to particular attackers, with the induction of one leading to suppression of the other. Here, we report a contrasting pattern in the woody perennialPopulus: positive SA–JA interplay.Using genetically engineered high SA lines of black poplar and wild‐type lines after exogenous hormone application, we quantified SA and JA metabolites, signalling gene transcripts, antifungal flavonoids and resistance to rust (Melampsora larici‐populina).Salicylic acid and JA metabolites were induced concurrently upon rust infection in poplar genotypes with varying resistance levels. Analysis of SA‐hyperaccumulating transgenic poplar lines showed increased jasmonate levels, elevated flavonoid content and enhanced rust resistance, but no discernible reduction in growth. Exogenous application of either SA or JA triggered the accumulation of the other hormone. Expression of pathogenesis‐related (PR) genes, frequently used as markers for SA signalling, was not correlated with SA content, but rather activated in proportion to pathogen infection.We conclude that SA and JA pathways interact positively in poplar resulting in the accumulation of flavonoid phytoalexins.