• Pelagiomicin A, a novel phenazine antibiotic from the marine bacterium Pelagiobacter variabilis, is unstable in water and small alcohols. The compound's major component, Pelagiomicin A, along with the properties of minor structures, were determined through spectroscopic data and synthesis. Pelagiomicin A demonstrates activity against both Gram-positive and Gram-negative bacteria and shows antitumor effects both in vitro and in vivo.