The Arctic Ocean is rapidly transforming due to climate change where decreasing sea ice has led to an increase in primary productivity. A main unknown of future primary production is the availability of nitrogen, which often is a limiting nutrient in the Arctic Ocean. Nitrogen fixation – the reduction of molecular nitrogen to bioavailable ammonia – has been detected in the Pacific Arctic despite previously being considered a process limited to tropical/subtropical oligotrophic waters. Prokaryotes performing nitrogen fixation are called diazotrophs and cover a wide range of cyanobacterial and non-cyanobacterial microorganisms. Knowledge of Arctic diazotrophs and nitrogen fixation is sparse, where especially the Eurasian Arctic Ocean is understudied with almost only putative diazotrophs detected to date. In this PhD thesis, the magnitude of pelagic nitrogen fixation in the Eurasian Arctic, the underpinning diazotroph communities, and potential environmental drivers were targeted. The overall aim was to contribute to deciphering whether nitrogen fixation is of relevance to the pan-Arctic nitrogen budget. Investigations took place over varying spatiotemporal scales where it was found that non-cyanobacterial diazotrophs dominate and are the likely contributors to nitrogen fixation in the Eurasian Arctic Ocean. Three key noncyanobacterial groups were identified and quantified (up to 105 and 103 nifH gene and transcript copies L-1, respectively), displaying either Arctic-endemic or more cosmopolitan distributions. Spatial and seasonal patterns of diazotroph community composition and abundances were identified, presenting distinct communities within regions of the Arctic largely shaped by freshwater influence. A sharp switch from photosynthesis-based (cyanobacterial: Candidatus Atelocyanobacterium thalassa) to likely heterotrophy-based (non-cyanobacterial) diazotrophy was detected when entering Arctic (Atlantic-originating) nutrientrich waters. Nitrogen fixation was measured to a maximum rate of 5.3 ± 3.4 nmol N L-1 d-1 in the marginal ice zone, contributing up to 9% of in-situ primary production in the central Arctic Ocean. Nitrogen fixation could be linked to primary production, inorganic nutrients, and meltwater influence, and was additionally detected in sea ice and deep-sea sediment. Taken together, nitrogen fixation is here reported widely over the Eurasian Arctic Ocean. Future conditions are expected to regionally be favourable for diazotrophy, where the input of new nitrogen via nitrogen fixation is hypothesised to increase with increasing primary production and an expanding seasonal ice zone.