| Summary: | This study aimed to assess the diversity and antimicrobial activity of cultivable bacteria associated with Vietnamese sponges. In total, 460 bacterial isolates were obtained from 18 marine sponges. Of these, 58.3% belonged to <i>Proteobacteria</i>, 16.5% to <i>Actinobacteria</i>, 18.0% to <i>Firmicutes</i>, and 7.2% to <i>Bacteroidetes</i>. At the genus level, isolated strains belonged to 55 genera, of which several genera, such as <i>Bacillus</i>, <i>Pseudovibrio</i>, <i>Ruegeria</i>, <i>Vibrio</i>, and <i>Streptomyces</i>, were the most predominant. Culture media influenced the cultivable bacterial composition, whereas, from different sponge species, similar cultivable bacteria were recovered. Interestingly, there was little overlap of bacterial composition associated with sponges when the taxa isolated were compared to cultivation-independent data. Subsequent antimicrobial assays showed that 90 isolated strains exhibited antimicrobial activity against at least one of seven indicator microorganisms. From the culture broth of the isolated strain with the strongest activity (<i>Bacillus</i> sp. M1_CRV_171), four secondary metabolites were isolated and identified, including cyclo(L-Pro-L-Tyr) (<b>1</b>), macrolactin A (<b>2</b>), macrolactin H (<b>3</b>), and 15,17-epoxy-16-hydroxy macrolactin A (<b>4</b>). Of these, compounds <b>2</b>-<b>4</b> exhibited antimicrobial activity against a broad spectrum of reference microorganisms.
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