Biological dinitrogen (N ₂ ) fixation is an important source of nitrogen (N) in low-latitude open oceans. The unusual N ₂ -fixing unicellular cyanobacteria (UCYN-A)/haptophyte symbiosis has been found in an increasing number of unexpected environments, including northern waters of the Danish Straight and Bering and Chukchi Seas. We used nanoscale secondary ion mass spectrometry (nanoSIMS) to measure ¹⁵ N ₂ uptake into UCYN-A/haptophyte symbiosis and found that UCYN-A strains identical to low-latitude strains are fixing N ₂ in the Bering and Chukchi Seas, at rates comparable to subtropical waters. These results show definitively that cyanobacterial N ₂ fixation is not constrained to subtropical waters, challenging paradigms and models of global N ₂ fixation. The Arctic is particularly sensitive to climate change, and N ₂ fixation may increase in Arctic waters under future climate scenarios.