In recent decades, global climate change has significantly threatened food production and quality. Salinity, drought, nutrient deficiencies, and extreme temperatures, often accompanied by prolonged dry periods, are among the most common challenges to be overcome. Consequently, there is an urgent need to develop formulations that can enhance crop growth and resistance to abiotic stresses. Biostimulants, known for stimulating natural plant processes, are applied to plants or soil to improve nutrient uptake and tolerance to abiotic stress. Recently, the formulation of biostimulants using by-products generated along the agri-food chain, such as seeds, leaves, peels, and kernels, has been proposed, thereby contributing to the circular economy. This systematic review aims to provide data on the extraction techniques necessary to obtain compounds from agri-food by-products. Techniques include pressurized liquid extraction, supercritical fluid extraction, ultrasound-assisted extraction, maceration, liquid–liquid extraction, and solid–liquid extraction. The review also examines the effects of these by-product-based biostimulants on plant growth and resistance to abiotic stress, anddiscusses the limitations of this novel approach. By focusing on the utilization of agri-food by-products in biostimulant formulations, this review highlights the potential benefits and challenges of integrating sustainable practices in agriculture to combat the adverse effects of climate change.