Nitrogen is a key element for life in the oceans. It controls primary productivity in many parts of the global ocean, consequently playing a crucial role in the uptake of atmospheric carbon dioxide. The nitrogen cycle is driven by complex biogeochemical transformations mediated by microorganisms, including classical processes such as nitrogen fixation, assimilation, nitrification, denitrification, and dissimilarity nitrate reduction to ammonia, as well as novel processes such as anaerobic ammonium oxidation, comammox and nitrite-driven anaerobic methane oxidation. The nitrogen cycle maintains the functioning of marine ecosystems and will be a crucial component in how the ocean responds to global environmental change. In this review, we summarize the current understanding of the marine microbial nitrogen cycle, its underlying biochemical and enzymatic reactions, the ecology and distribution of the microorganisms involved, and the main impacts of anthropogenic activities.