This study presents basin-wide, full-depth profiles of nitrate δ15N (vs. Air) and δ18O (vs. Vienna Standard Mean Ocean Water, VSMOW) in the Mediterranean Sea. Our results reveal a consistent 15N depletion across the entire Mediterranean Sea in comparison to the global ocean, with significantly lower nitrate δ15N values in the eastern basin (2.2 ± 0.2‰) than in the western basin (2.9 ± 0.1‰). In contrast, there is no significant difference in nitrate δ18O between the two basins (2.2 ± 0.3‰ and 2.1 ± 0.2‰, respectively). These observations point to an external supply of low-δ15N N to the Mediterranean Sea, accumulating as nitrate. This supply is diluted by the Atlantic inflow, creating an east-to-west gradient in nitrate δ15N. Earlier studies have attributed the external low-δ15N N supply to either N2 fixation or atmospheric deposition of anthropogenic N. A four-box model reveals that, given the water residence time of 120–170-years in the Mediterranean, a modest input rate of 1–3 Tg N yr-1 from either of these two sources or their combination is adequate to produce the observed low δ15N of Mediterranean nitrate. Additionally, partial degradation of dissolved organic nitrogen imported from the Atlantic is another possible source of the low-δ15N nitrate in the Mediterranean. Distinguishing among these sources will be aided by reconstruction of Mediterranean nitrate δ15N through time, using either time-series data of nitrate δ15N or paleoceanographic proxies.