Background: Soya bean is a major food allergen in children. Component-resolved diagnostics has improved the accuracy of diagnosing immunoglobulin E (IgE)-mediated food allergies.Objective: We aimed to develop a novel component for the diagnosis of soya bean allergy using recombinant technology.Methods: Japanese paediatric patients with suspected soya bean allergy (n = 91) were included, and symptomatic (n = 40) and asymptomatic (n = 51) cases were divided through oral food challenge testing. Specific IgE (sIgE) antibodies to each recombinant allergen component were analysed by enzyme-linked immunosorbent assay, and the diagnostic performances of the components were assessed by area under the receiver operating characteristic curves (AUC).Results: Among the recombinant components, sIgE antibody levels to Gly m 8 showed the highest AUC (0.706). A combination of Gly m 8 and alpha' subunit of Gly m 5, improved the diagnostic performance of the single components. Moreover, the N-terminal extension region of alpha' subunit of Gly m 5, which has low cross-reactivity among the vicilins, showed higher diagnostic performance (AUC 0.695) than the full-length alpha' subunit of Gly m 5 (AUC 0.613). Based on these findings, we developed a fusion protein of Gly m 8 plus the extension region of alpha' subunit of Gly m 5; this fusion protein was very efficient for diagnosing soya bean allergy (AUC 0.801).Conclusion: A fusion protein of Gly m 8 and the extension region of alpha' subunit of Gly m 5 could potentially diagnose soya bean allergy in paediatric patients. Fusion proteins may be useful for producing novel allergen components with improved diagnostic value.