The uptake rates of ammonium and nitrate through the leaves and roots, the leaf–root interactions in the nitrogen uptake and the internal translocation of incorporated nitrogen were simultaneously investigated in the seagrass Zostera noltii. Leaf and root uptake rates, which were measured using two-compartment polyethylene chambers that physically separated the leaves from below-ground plant parts, were quantified based on tissue incorporation of 15N-labeled ammonium and nitrate. The maximum leaf uptake rates (Vmax) of ammonium were 100 times higher than those of nitrate. Both Vmax and affinity for ammonium were one order of magnitude higher in the leaves (28.3–31.9 μmol g−1 DW h−1 and 0.93–0.99, respectively) than in the roots (2.3–3.0 μmol g−1 DW h−1 and 0.06–0.08, respectively). The uptake of ammonium and nitrate by one plant part did not affect the uptake of the other plant part, and no translocation of inorganic nitrogen was detected between plant parts. The 15N enrichment detected in the rhizomes suggests either a direct uptake of inorganic nitrogen or its transference from the roots. The estimated total inorganic nitrogen uptake of Z. noltii (645 μmol m−2 h−1) in the peak production season (spring) under typical nutrient concentrations, using the rates obtained during the surge uptake phase, exceeded by 3-fold the species estimated nitrogen requirement for growth (236 μmol N m−2 h−1). However, using the stabilized values of the uptake rates obtained after several hours of incubation, the estimated whole-plant nitrogen budget (215 μmol m−2 h−1) was slightly lower than the total nitrogen requirement for growth. We conclude that the growth of Z. noltii in Ria Formosa lagoon is not limited, or is only slightly limited, by nitrogen.