Salinity, one of the major abiotic stresses in plants, significantly hampers germination,
photosynthesis, biomass production, nutrient balance, and yield of staple crops. To mitigate the
impact of such stress without compromising yield and quality, sustainable agronomic practices are
required. Among these practices, seaweed extracts (SWEs) and microbial biostimulants (PGRBs) have
emerged as important categories of plant biostimulants (PBs). This research aimed at elucidating the
effects on growth, yield, quality, and nutrient status of two Greek tomato landraces (‘Tomataki’ and
‘Thessaloniki’) following treatments with the Ascophyllum nodosum seaweed extract ‘Algastar’ and the
PGPB ‘Nitrostim’ formulation. Plants were subjected to bi-weekly applications of biostimulants and
supplied with two nutrient solutions: 0.5 mM (control) and 30 mM NaCl. The results revealed that
the different mode(s) of action of the two PBs impacted the tolerance of the different landraces, since
‘Tomataki’ was benefited only from the SWE application while ‘Thessaloniki’ showed significant
increase in fruit numbers and average fruit weight with the application of both PBs at 0.5 and 30 mM
NaCl in the root zone. In conclusion, the stress induced by salinity can be mitigated by increasing
tomato tolerance through the application of PBs, a sustainable tool for productivity enhancement,
which aligns well with the strategy of the European Green Deal.