<p>Iminium-catalysed cycloaddition is one of the most prominent examples of organocatalysis, yet a biological counterpart has not been reported, despite the widespread occurrence of iminium adducts in enzymes. Here we present biochemical, structural and computational evidence for iminium catalysis by the natural Diels–Alderase SdnG, which catalyses norbornene formation in sordarin biosynthesis. A Schiff-base adduct between the ε-nitrogen of active site K127 and the aldehyde group of the enal dienophile is revealed by structural analysis and captured under catalytic conditions via borohydride reduction. This Schiff-base adduct positions the substrate into near-attack conformation and decreases the transition-state barrier of Diels–Alder cyclization by 8.3 kcal mol⁻¹ via dienophile activation. A hydrogen-bond network consisting of a catalytic triad is proposed to facilitate the proton transfer required for iminium formation. This work establishes an intriguing mode of catalysis for Diels–Alderases and points the way to the design of iminium-based (bio)catalysts.</p><p></p>