Salmonids are important sources of protein for a large proportion of the human population. Interaction between the gut microbiota and host has been shown to affect the host phenotype in mammals, but relatively little is known about microbiota-host interaction in fish. Mycoplasma species are a major constituent of the gut microbiota of salmonids, often representing the majority of microbial cells. Despite the frequent reported dominance of salmonid-related Mycoplasma species, little is known about the phylogenomic placement, functions and potential evolutionary relationships with their salmonid hosts. In this study, we utilise 2.9 billion metagenomic reads generated from 12 samples from three different salmonid host species to I) characterise and curate the first metagenome-assembled genomes (MAGs) of Mycoplasma dominating the intestines of three different salmonid species, II) establish the phylogeny of these salmonid candidate Mycoplasma species using known Mycoplasma genomes, III) perform a comprehensive pangenomic analysis of Mycoplasma, IV) decipher the putative functionalities of the salmonid MAGs and reveal specific functions expected to benefit the host. Our data provide a basis for future studies examining the composition and function of the salmonid microbiota, with a potential for being further exploited in order to increase animal health and growth in aquaculture.