This study investigated the effects of nitrate (NO₃⁻) and ammonium (NH₄⁺) nutrition on the growth and physiology of Medicago sativa L. under salt stress. Plants were grown for 40 days in sand–peat (2:1) medium supplied with either 3 mM NO₃⁻ or 3 mM NH₄⁺, with or without 100 mM NaCl. Growth, ion content, chlorophyll concentration, and total polyphenol content were assessed in shoots. In control conditions, NO₃⁻ nutrition promoted higher biomass production than NH₄⁺. Salt stress reduced growth by 68% in NO₃⁻-fed plants but increased biomass by 24% in NH₄⁺-fed plants. NH₄⁺ nutrition under salinity lowered Na⁺ accumulation in shoots and enhanced chlorophyll and polyphenol contents compared with NO₃⁻. These findings indicate that M. sativa is more tolerant to salinity when supplied with NH₄⁺, likely due to reduced Na⁺ uptake and improved antioxidant capacity. NH₄⁺ fertilization could thus mitigate salt stress effects in alfalfa cultivation.