Hordeum maritimum With. (= H. marinum Huds. subsp. marinum, 2n = 14) is a wild cereal present in the saline depressions of the Soliman and Kelbia Sebkhas, which contributes significantly to annual biomass production in Tunisia. This species is able to tolerate high NaCl concentrations at the seedling stage without showing symptoms of toxicity; however, the tolerance strategy mechanisms of this plant have not yet been unravelled. Our metabolite analysis, performed on leaves of H. maritimum during extended stress in comparison with Hordeum vulgare L. cv. Lamsi, has revealed an adaptive response of the wild species based on a different temporal accumulation pattern of ions and compatible metabolites. Further, wild and cultivated genotypes with contrasting salt-tolerant behaviour display different pattern of metabolites when salt stress is prolonged over 2 weeks. In particular, when exposed to up to 3 weeks of 200 mM NaCl salt stress, H. maritimum is able to maintain lower leaf concentrations of sodium and chloride, and higher concentrations of potassium compared with H. vulgare. This likely restricts sodium entry into plants at the root level, and uses the toxic ions, glycine betaine and low levels of proline for osmotic adjustment. Under prolonged stress, the accumulation of proline increases, reaching the highest levels in concomitance with the decrease of potassium to sodium ratio, the increase of hydrogen peroxide and decrease of chlorophylls. The modulation of proline accumulation over time can be interpreted as an adaptive response to long-term salinity. Moreover, once synthetised glycine betaine is transported but not metabolised, it can contribute together with proline to osmotically balance H. maritimum leaves and protect them from oxidative stress. The 2–3 week delay of H. maritimum in showing the symptoms of stress and damages compared with H. vulgare could be important in the survival of plants when soil salinity is not a permanent condition, but just a transient state of stress.