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Highlight The class II diterpene synthase that catalyzes the first committed reaction of the virtually unknown biosynthetic pathway of salvinorin A in Salvia divinorum is identified and the structural basis for product specificity of this enzyme is elucidated., Salvia divinorum (Lamiaceae) is an annual herb used by indigenous cultures of Mexico for medicinal and ritual purposes. The biosynthesis of salvinorin A, its major bioactive neo-clerodane diterpenoid, remains virtually unknown. This investigation aimed to identify the enzyme that catalyzes the first reaction of salvinorin A biosynthesis, the formation of (–)-kolavenyl diphosphate [(–)-KPP], which is subsequently dephosphorylated to afford (–)-kolavenol. Peltate glandular trichomes were identified as the major and perhaps exclusive site of salvinorin accumulation in S. divinorum. The trichome-specific transcriptome was used to identify candidate diterpene synthases (diTPSs). In vitro and in planta characterization of a class II diTPS designated as SdKPS confirmed its activity as (–)-KPP synthase and its involvement in salvinorin A biosynthesis. Mutation of a phenylalanine into histidine in the active site of SdKPS completely converts the product from (–)-KPP into ent-copalyl diphosphate. Structural elements were identified that mediate the natural formation of the neo-clerodane backbone by this enzyme and suggest how SdKPS and other diTPSs may have evolved from ent-copalyl diphosphate synthase.