Organic soil amendments have become key components of sustainable and organic agriculture because of their capacity to improve soil fertility while simultaneously enhancing environmental sustainability. Unlike conventional mineral fertilizers, these materials contribute to long-term soil functionality by increasing soil organic matter, stimulating microbial activity, improving nutrient cycling, and strengthening soil structure. This review synthesizes current scientific evidence regarding the classification, mechanisms of action, and agronomic importance of organic soil amendments, with particular emphasis on their effects on soil health, soil organic carbon sequestration, and climate change mitigation. The available literature indicates that amendment effectiveness depends on multiple interacting factors, including amendment type, soil characteristics, climatic conditions, and management practices, highlighting the need for site-specific management strategies rather than universal recommendations. Furthermore, this review critically discusses the environmental benefits associated with nutrient recycling, waste valorization, greenhouse gas mitigation, and ecosystem restoration, while also addressing current limitations related to amendment quality, field performance, and implementation. Overall, organic soil amendments represent fundamental tools for improving agricultural sustainability, supporting climate-resilient farming systems, and promoting integrated soil management approaches capable of simultaneously enhancing productivity, ecosystem functioning, and long-term environmental quality.