Salinity stress remains one of the most severe abiotic constraints limiting global crop productivity, particularly in arid and semi-arid regions. Seaweed-derived biostimulants have gained attention as sustainable tools to enhance plant resilience under saline conditions due to their diverse and complex bioactive composition. This review examines how taxonomic origin and extraction-dependent compositional variability influence the efficacy of seaweed extracts in mitigating salinity stress. Particular attention is given to salinityspecific physiological and biochemical mechanisms, including ion homeostasis regulation, antioxidant system activation, osmolyte accumulation, hormonal modulation, and emerging molecular responses. Rather than focusing solely on reported benefits, this review adopts a constraint-oriented perspective by synthesizing how algal taxonomy and extraction processes shape the reproducibility and magnitude of stress tolerance outcomes. Comparative evidence is used to connect the dominant chemical classes of brown, red, and green algae with distinct salinity-relevant physiological and molecular responses across different crop systems. Furthermore, crop-specific variability, standardization challenges, and biochemical limitations that influence field-level performance are critically discussed. This integrative approach aims to clarify mechanistic understanding and support more targeted application of seaweed-derived biostimulants for improving abiotic stress tolerance under increasing salinity pressures.