Abiotic stresses, intensified by climate change, account for up to 70% of global crop yield losses, posing a critical threat to food security and sustainable agriculture. Conventional reliance on synthetic agrochemicals has increased productivity but also caused soil degradation, environmental toxicity, and biodiversity loss. Biostimulants, including seaweed and plant extracts, protein hydrolysates, humic substances, and beneficial microorganisms, are emerging as eco-friendly alternatives that enhance plant resilience to drought, salinity, temperature extremes, and heavy metal toxicity. This review synthesizes current evidence on the biochemical and molecular mechanisms through which biostimulants mitigate stress, focusing on activation of antioxidant defense, phytohormone regulation, osmolyte accumulation, and modulation of gene expression. A comparative analysis highlights the differential effectiveness of biostimulant categories across stress types and identifies synergistic benefits of combined applications. Despite promising results, challenges remain, including inconsistent field performance, a lack of mechanistic standardization, and regulatory ambiguity. Future prospects lie in integrating biostimulants with multi-omics tools, precision agriculture, and sustainable production strategies to optimize their efficacy and scalability. By critically examining recent advances and knowledge gaps, this review provides a roadmap for harnessing biostimulants as next-generation tools to improve crop productivity and agricultural sustainability under climate change.