A new cross-linked polyamine alkaloid, named tenuilobine (1), was isolated from the leaves of Oncinotis tenuiloba (Apocynaceae), the structure was established by spectroscopic means. It is the first natural alkaloid containing spermine and spermidine as polyamine parts. Presumably, compound (1) is involved in the biosynthesis of the macrocyclic spermidine alkaloids with lactam groups.