This chapter focuses on small-molecule (<3 kDa) antibacterial compounds from marine bacteria. In addition to the small-molecule antibiotics, antibiotic macromolecules including amino acid oxidases and glycoproteins, have also been isolated from marine bacteria. Rediscoveries of known antibiotics are common among Actinobacteria, including Streptomyces, Micromonospora, and Salinispora, but also occur in evolutionary distant microbes with distinct niches. Examples of cosmopolitan antibiotics in marine bacteria are listed. The phenomenon of cosmopolitan antibiotics affects natural product research, since even extensive microbiological and chemical studies of yet uncharacterized microbes can result in rediscoveries. Marine bacteria are a promising resource of novel antibacterial compounds to overcome evolving resistances toward traditional antibiotics. The discovery of marine bacterial antibiotics will benefit from more sophisticated isolation, cultivation, and screening techniques in combination with genomic analyses.