Marine N
2
fixation supports a significant portion of oceanic primary production by making N
2
bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N
2
fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N
2
fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N
2
fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N
2
fixation is best correlated to phosphorus availability and chlorophyll-
a
concentration. Globally, intense N
2
fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N
2
fixation flux to be 16.7 Tg N. This study broadens the biogeography of N
2
fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles.
The geographical distribution and controlling factors of marine N
2
fixation are understudied. Here the authors find increasing rates of N
2
fixation from the Sargasso Sea to the coastal waters of North America, driven primarily by cyanobacterial diazotrophs and best correlated with phosphorus availability and chlorophyll-
a
concentrations.