Soil salinity is deleterious for the growth and biological N2 fixation of alfalfa (Medicago sativa L.), but nitrogen fertilization has been beneficial in some cases. This study was conducted to investigate the potential benefit of N fertilization on alfalfa growth and N2 fixation under saline conditions. Solutions of sodium chloride (NaCl) and hydrous calcium chloride (CaCl2.2H2O) were added to induce ECe (electrical conductivity of the saturated paste extract) targeted at 0.5, 5, 10, and 15 dS m−1 in a sandy loam soil. 15N-labeled ammonium nitrate (NH4NO3) was added to the soil at 30, 60, and 120 mg N kg−1 soil. Alfalfa shoot and root dry weights, root nodule counts, shoot total nitrogen (N), potassium (K) and sodium (Na+), and the amount of N derived from air (%Ndfa) were determined. Shoot Na concentration increased, and plant biomass, shoot N and K concentration, root nodules, and nitrogen Ndfa decreased as soil salinity increased. At all salinity levels, the highest plant biomass, shoot total N content, and K/Na ratio were recorded at 120 mg N kg−1 soil. Root nodulation and %Ndfa were highest at 60 mg N kg−1 soil under nonsaline conditions while under saline conditions, the highest nodulation and %Ndfa were recorded at 30 mg N kg−1 soil. Plant biomass, nodule number, and %Ndfa were greater in the third cut than in the first cut. Addition of nitrogen fertilizers seemed to have a positive effect in alleviating salt stress in alfalfa and maintaining shoot biomass production, but N fertilization may limit the plant’s ability to fix atmospheric N.