The potential of plant biodiversity to support food production and preserve ecosystem services is not fully explored and exploited. Neglected and underutilised species (NUS) for example can represent an extremely interesting economic source in marginal areas of Mediterranean, where degradation, drought and also salinization might impoverish farmer options. NUS might present many environmental advantages, one of the most interesting being their limited water requirements. In the VENUS PRIMA project we investigate the water footprint of these plants to demonstrate their positive aspects compared with most common and diffused crops. However, in order to estimate the water footprint we need to estimate green, blue and grey water. The green water footprint (GWF) of NUS has never been quantified under field conditions, owing to the absence of crop-specific coefficients (Kc) required by the standard FAO-56 framework. To meet our objectives we decided to take advantage of δ¹³C-based isotopic techniques to calculate the intrinsic water use efficiency (iWUE) and from there the GWF for six Mediterranean halophyte NUS — Salsola soda, Atriplex halimus, Crithmum maritimum, Beta maritima, Salicornia europaea, and Suaeda maritima — cultivated under rainfed conditions on marginal saline soils at two Italian field sites (CONSAE/Veneto and RINCI/Marche) during the 2025 growing season, within the VENUS project. A Kc-independent δ¹³C-based isotopic pipeline was applied, deriving iWUE from leaf carbon isotope discrimination via the Farquhar model and scaling to GWF through biomass water use efficiency and NASA POWER MERRA-2 climate reanalysis data. A Picarro δ¹³C analyser was used to estimate the isotopic discrimination of plant samples, while their biomass was used to compute the final GWF. NUS showed iWUE values of 181–262 µmol CO₂ mol⁻¹ H₂O for C4 species and 37–83 µmol CO₂ mol⁻¹ H₂O for C3 species, corresponding to a GWF of 5.76–7.28 L kg⁻¹ and 12.13–40.95 L kg⁻¹, respectively. These values were substantially lower than the 238–384 L kg⁻¹ total water footprint of conventional leafy vegetables, while for all NUS species, blue and grey water footprints were zero based on their management requirements, showing the extremely good performance of NUS for the water footprint sustainability indicator. Macronutrient data indicate competitive dietary fibre contents for Beta maritima and Crithmum maritimum relative to conventional leafy vegetables, with published micronutrient data further supporting the nutritional relevance of several species. All plants present interesting market applications. These results provide the quantitative evidence base needed to support the inclusion of Mediterranean NUS in sustainable agriculture in accordance with EU agricultural directives of sustainability and competitiveness.