Sustainable biopolymer-based composites have emerged as essential alternatives to conventional petroleum-derived polymers due to the urgent need to reduce environmental pollution and promote renewable material cycles. Their development is closely associated with global sustainability goals, circular-economy frameworks, and green manufacturing practices. Biopolymers derived from natural, renewable resources such as plant fibers, microbial fermentation, and agro-industrial residues have demonstrated remarkable potential for high-performance material applications. These composites offer advantages including biodegradability, lightweight characteristics, improved mechanical behavior, and reduced carbon footprint. The rapid evolution of advanced processing techniques—such as reactive extrusion, melt blending, and additive manufacturing—has enabled the fabrication of customized biocomposite structures with enhanced functionality. Recent developments also show the integration of biopolymers into emerging fields, including energy storage, biomedical engineering, flexible electronics, and environmental remediation. Despite their progress, challenges remain regarding material stability, moisture sensitivity, cost competitiveness, and end-of-life management. This review synthesizes current advancements in feedstocks, material design, structural characteristics, applications, and processing strategies. It also outlines the technological opportunities and future research directions necessary to transform biopolymer composites into mainstream sustainable engineering materials.