IN BRIEF: Reproductive microbiomes contribute to the successful embryogenesis of offspring but are poorly studied in non-mammalian species that exhibit pregnancy. This study characterises the male pregnant seahorse brood pouch microbiome and identifies potential microbial maternal contributions to the pouch, providing insights into the sources and adaptive value of the embryonic microbial environment. ABSTRACT: Seahorses demonstrate an unusual reproductive strategy, in which males incubate embryos inside a complex 'brood pouch' until parturition, analogous to mammalian viviparity. In many species, a 'normal' reproductive microbiome ensures successful embryogenesis and enables parents (usually mothers) to provide their offspring with their initial microbiome. In male-pregnant seahorses, embryos may receive microbiomes from both parents: from the paternal brood pouch and from the maternal eggs. Using the pot-bellied seahorse (Hippocampus abdominalis), we employed 16S rRNA sequencing to explore the reproductive microbiome. We aimed to compare the microbiome of the male pregnant pouch to the male non-pregnant pouch and external skin, and to identify bacterial taxa found exclusively in the pregnant pouch that could be derived maternally from the microbiome of eggs. Our findings demonstrate that the pregnant brood pouch microbiome is compositionally distinct from the non-pregnant pouch and external skin. The pouch microbiome also has characteristics of resistance to colonisation by pathogens, including a low species richness, high species evenness and diversity and very low abundance of Vibrio, a genus that includes fish skin pathogens. Thirteen bacterial taxa appear exclusively in the pregnant pouch, relative to the non-pregnant pouch, and seven of these overlapped with taxa present in or on the eggs. The possible supplementation of brood pouch microbiome with egg-associated micro-organisms hints at a maternal microbial contribution to male pregnancy. This characterisation of the pregnant seahorse pouch microbiome provides a platform for further research into its function and possible adaptive value during male pregnancy.