Menaquinone (MK, vitamin K2) is a lipid-soluble molecule that participates in the bacterial electron transport chain. In mammalian cells, MK functions as an essential vitamin for the activation of various proteins involved in blood clotting and bone metabolism. Recently, a new pathway for the biosynthesis of this cofactor was discovered in Streptomyces coelicolor A3(2) in which chorismate is converted to aminofutalosine in a reaction catalyzed by MqnA and an unidentified enzyme. Here, we reconstitute the biosynthesis of aminofutalosine and demonstrate that the missing enzyme (aminofutalosine synthase, MqnE) is a radical SAM enzyme that catalyzes the addition of the adenosyl radical to the double bond of 3-[(1-carboxyvinyl)oxy]benzoic acid. This is a new reaction type in the radical SAM superfamily.