The rising global population has intensified the need for sustainable agricultural practices to ensure food security without degrading the environment. Chemical fertilizers, while effective in enhancing crop productivity, have contributed to soil degradation, water pollution, and ecological imbalance. Biofertilizers are the formulations containing live or dormant microorganisms emerged as eco-friendly alternatives that enhance nutrient availability and improve soil health. These beneficial microbes, including nitrogen-fixers (e.g., Rhizobium, Azospirillum, Azotobacter), phosphate and potassium solubilizing bacteria, mycorrhizal fungi, and plant growth-promoting rhizobacteria (PGPR), play crucial roles in nutrient cycling, stress tolerance, and crop productivity. Symbiotic and free-living nitrogen-fixers significantly contribute to atmospheric nitrogen conversion into plant-usable forms, while phosphate and potassium solubilizers mobilize essential nutrients locked in soil. Additionally, biofertilizers improve plant resilience to environmental stresses such as drought and salinity, promote root growth, and increase crop yields by 10–50%. Biofertilizers also offer a sustainable solution to micronutrient deficiencies like sulfur and zinc, improving plant nutrition and reducing dependence on synthetic inputs. Their use aligns with the principles of organic farming and climate-smart agriculture, reducing greenhouse gas emissions and conserving nonrenewable resources such as phosphate rock. Scientific evidence supports the efficacy of biofertilizers in enhancing nitrogen and phosphorus use efficiency, improving crop yield, and maintaining long-term soil fertility. This chapter highlights the various types of biofertilizers, their mechanisms of action, and their vital role in sustainable agriculture as a green alternative to chemical fertilizers, ultimately contributing to food security and environmental sustainability.