We identified a unique enzyme that catalyzes the oxidocyclization of cannabigerolic acid to cannabidiolic acid (CBDA) in Cannabis sativa L. (CBDA strain). The enzyme, named CBDA synthase, was purified to apparent homogeneity by a four-step procedure: ammonium sulfate precipitation followed by chromatography on DEAE-cellulose, phenyl-Sepharose CL-4B, and hydroxylapatite. The active enzyme consists of a single polypeptide with a molecular mass of 74 kDa and a pI of 6.1. The NH2-terminal amino acid sequence of CBDA synthase is similar to that of Delta1-tetrahydrocannabinolic-acid synthase. CBDA synthase does not require coenzymes, molecular oxygen, hydrogen peroxide, and metal ion cofactors for the oxidocyclization reaction. These results indicate that CBDA synthase is neither an oxygenase nor a peroxidase and that the enzymatic cyclization does not proceed via oxygenated intermediates. CBDA synthase catalyzes the formation of CBDA from cannabinerolic acid as well as cannabigerolic acid, although the kcat for the former (0.03 s-1) is lower than that for the latter (0.19 s-1). Therefore, we conclude that CBDA is predominantly biosynthesized from cannabigerolic acid rather than cannabinerolic acid.