New bioavailable nitrogen (N) from biological nitrogen fixation (BNF) is critical for the N budget and productivity of marine ecosystems. Nitrogen-fixing organisms typically inactivate BNF when less metabolically costly N sources, like ammonium (NH4+), are available. Yet, several studies observed BNF in benthic marine sediments linked to anaerobic sulfate-reducing bacteria (SRB) and fermenting firmicutes despite high porewater NH4+;concentrations (10-1,500 μM), making the importance of and regulating controls on benthic BNF unclear. Here, we evaluate BNF sensitivity to NH4+ in model anaerobic diazotrophs, the sulfate-reducer Desulfovibrio vulgaris var. Hildenborough and fermenter Clostridium pasteurianum strain W5; in sulfate-reducing sediment enrichment cultures, and in sediment slurry incubations from three Northeastern salt marshes (USA). BNF in sulfate-reducing cultures and sediments is highly sensitive to external NH4+, with a threshold for BNF inhibition of [NH4+] < 2 μM in cultures and < 9 μM in sediment slurries. The prevalence of SRB-like sequences in sediment-derived nitrogenase (nifH) genes and transcripts in this and other studies of benthic BNF along with an analysis of benthic NH4+ porewater data suggests a broad applicability of the inhibition thresholds measured here and the confinement of benthic BNF to surficial sediments. The timing of inhibition, fast NH4+ drawdown, and sediment heterogeneity are factors that can complicate studies of benthic BNF sensitivity to NH4+. We propose a simple theoretical framework based on the affinity of the NH4+ transporter to explain NH4+ control of BNF and improve biogeochemical models of N cycling.