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 (NH 4 +), are available. Yet, several studies have observed BNF in benthic marine sediments linked to anaerobic sulfate-reducing bacteria and fermenting firmicutes despite high porewater NH 4 + concentrations (10–1,500 μM). To better understand the regulating controls and importance of benthic marine BNF, we evaluate BNF sensitivity to NH 4 + in benthic diazotrophs using incubations of increasing biogeochemical complexity. BNF by cultures of model anaerobic diazotrophs (sulfate-reducer Desulfovibrio vulgaris var. Hildenborough, fermenter Clostridium pasteurianum strain W5), sulfate-reducing sediment enrichment cultures, and sediments from three Northeastern salt marshes (USA) is highly sensitive to external NH 4 + . BNF is inhibited by NH4 + beyond an apparent threshold [NH4 + ] of 2 μM in liquid cultures, most closely reflecting the true cellular sensitivity of BNF to NH4 +. Sediment slurries exhibited an apparent threshold [NH 4 + ] of 9 μM. Consistent with other studies, we find SRB-like nitrogenase (nifH) gene and transcripts are prevalent in sediments. Our survey of porewater NH 4 + data from diverse sediments suggests the broad applicability of inhibition thresholds measured here and confinement of benthic BNF to surficial sediments. Variations in BNF inhibition timing, NH 4 + uptake rate, sediment composition, and biophysics could affect measurements of the apparent sensitivity of benthic BNF to NH 4 + . We propose NH4 + transporter affinity as a fundamental mechanistic constraint on NH4 + control of cellular BNF to improve biogeochemical models of N cycling.