Aims: To evaluate the diversity and antimicrobial activity present among Pseudovibrio spp. isolated from marine sponges. Methods and Results: Seventy-three bacterial isolates from the marine sponges Polymastia boletiformis, Axinella dissimilis and Haliclona simulans were identified as Pseudovibrio spp. using phylogenetic analysis of 16S rRNA gene sequences. Genetic diversity among these isolates was estimated using random amplification of polymorphic DNA (RAPD), and 33 RAPD types were identified among the 73 Pseudovibrio isolates. These Pseudovibrio spp. were assayed for the production of compounds with antimicrobial activity against various clinically relevant pathogens. Sixty-two (85%) of the isolates showed activity against at least one of the pathogens tested, including Escherichia coli, Salmonella enterica serotype Typhimurium, methicillin-resistant Staphylococcus aureus (MRSA), and Clostridium difficile. PCR screens of the Pseudovibrio isolates also revealed the presence of potential antibiotic-producing polyketide synthase genes. Conclusions: Marine sponges harbour a diverse population of Pseudovibrio spp., the majority of which demonstrate antimicrobial activity. The identification of several different antimicrobial activity spectra suggests that the Pseudovibrio isolates may produce a suite of antimicrobial compounds. Significance and Impact of the Study: This is the first study in which an extended population of Pseudovibrio isolates from marine sponges has been analysed and establishes the little-studied Pseudovibrio as a potentially important genus in the search for antimicrobial compounds of clinical relevance.