Nitrite-oxidizing bacteria (NOB) are ubiquitous and abundant microorganisms that play key roles in global nitrogen and carbon biogeochemical cycling. Despite recent advances in understanding NOB physiology and taxonomy, there are currently very few cultured NOB or representative NOB genome sequences from marine environments. In this study, we employed enrichment culturing and genomic approaches in order to shed light on the phylogeny and metabolic capacity of marine NOB. We successfully enriched two marine NOB (designated MSP and DJ) and obtained a high-quality metagenome-assembled genome (MAG) from each organism. The maximum nitrite oxidation rates of the MSP and DJ enrichment cultures were 13.8 (0.1 mM nitrite as optimum) and 30.0 µM (0.3 mM nitrite) nitrite per day, respectively. Each enrichment culture exhibited different tolerance to varying nitrite and salt concentrations. Based on phylogenomic position and overall genome relatedness indices, both NOB MAGs were proposed as novel taxa within the Nitrospinota and Nitrospirota phyla. Functional predictions indicated that both NOB MAGs shared many highly conserve metabolic features with other NOB. Both NOB MAGs encoded proteins for hydrogen and organic compound metabolism and defense mechanisms for oxidative stress. Additionally, these organisms may have the genetic potential to produce cobalamin (an essential enzyme cofactor that is limiting in many environments) and thus may play an important role in recycling cobalamin in marine sediment. Overall, this study appreciably expands our understanding of the Nitrospinota and Nitrospirota phyla, and suggests that these NOB may play important biogeochemical roles in marine habitats. [ABSTRACT FROM AUTHOR]