A siderophore, named bisucaberin B, was isolated from Tenacibaculum mesophilum bacteria separated from a marine sponge collected in the Republic of Palau. Using spectroscopic and chemical methods, the structure of bisucaberin B ( 1) was clearly determined to be a linear dimeric hydroxamate class siderophore. Although compound 1 is an open form of the known macrocyclic dimer bisucaberin ( 2), and was previously described as a bacterial degradation product of desferrioxamine B ( 4), the present report is the first description of the de novo biosynthesis of 1. To the best of our knowledge, compound 1 is the first chemically characterized siderophore isolated from a bacterium belonging to the phylum Bacteroidetes.