<jats:title>Abstract</jats:title><jats:p>Some bacteria swarm under some circumstances; they move rapidly and collectively over a surface. In an effort to understand the molecular signals controlling swarming, we isolated two bacterial strains from the same red seaweed, <jats:italic>Vibrio alginolyticus</jats:italic> B522, a vigorous swarmer, and <jats:italic>Shewanella algae</jats:italic> B516, which inhibits <jats:italic>V. alginolyticus</jats:italic> swarming in its vicinity. Plate assays combined with NMR, MS, and X‐ray diffraction analyses identified a small molecule, which was named avaroferrin, as a potent swarming inhibitor. Avaroferrin, a previously unreported cyclic dihydroxamate siderophore, is a chimera of two well‐known siderophores: putrebactin and bisucaberin. The sequenced genome of <jats:italic>S. algae</jats:italic> revealed avaroferrin’s biosynthetic gene cluster to be a mashup of putrebactin and bisucaberin biosynthetic genes. Avaroferrin blocks swarming through its ability to bind iron in a form that cannot be pirated by <jats:italic>V. alginolyticus</jats:italic>, thereby securing this essential resource for its producer.