To manage the adverse effects of garbage pollution and avoid using chemicals, a natural extract of seafood shells was obtained and explored for its beneficial role. Physical characterization highlighted that its active compounds correspond to chitin and its derivative, chitosan. The ability of the extracted biostimulant to foster tomato tolerance was tested on drought-stressed plants. Along with changes in morphological parameters, the accumulation of chlorophyll and carotenoids was improved. The biostimulant also mediates the accumulation of osmoprotectants and an increased leaf water content. Furthermore, the biostimulant effectively promotes tolerance by increasing drought-stress SIERF84 Transcription factor and decreasing both SIARF4 and SlWRKY81 transcript levels, which in turn, mediates stomatal closure. In addition, the up-regulation of key genes related to NO3− uptake (NTR1.1/2) and assimilation (NR) coupled with the downregulation of ammonium transporters’ genes (AMT1.1/2), allowed the uptake of NO3− over NH4+ in the tolerant genotype which is likely to be associated with drought tolerance. Overall, the biostimulant was effective in alleviating water stress and showed similar effects to commercial chitosan. Besides the benefits of a circular economy framework, this biostimulant-based approach is innovative to promote a sustainable eco-agriculture, in the face of persistent water scarcity.