ScopeEnglish walnut (Juglans regia) belongs to the most important allergenic tree nuts. Co‐sensitization with birch (Betula verrucosa) pollen has been reported. We aimed to identify a walnut allergen homologous to the major birch pollen allergen Bet v 1.Methods and resultsA cDNA encoding a Bet v 1‐homologous allergen (Jug r 5) in walnut kernels was cloned by RT‐PCR. Jug r 5 was expressed in Escherichia coli, purified by column chromatography and characterized by circular dichroism spectroscopy. Specific IgE levels to walnut, Bet v 1, and Jug r 5 in birch pollen allergics (n = 16) with concomitant walnut allergy were measured by ImmunoCAP: 44% of the patients were tested positive to walnut while 94% were reactive to Jug r 5, and 100% to Bet v 1. Jug r 5 and Bet v 1 allergens showed bidirectional IgE cross‐reactivity by competitive ELISA and were capable of inducing histamine release from effector cells. Immunoblot competition experiments demonstrated the presence of IgE‐reactive Jug r 5 in walnut extract, but at low levels.ConclusionA Bet v 1‐like allergen was identified in walnut. Diagnostic use of Jug r 5 will compensate for the low sensitivity of walnut extract for patients with birch pollen associated walnut allergy.