Citrus waste valorization represents an integrated and sustainable approach for converting abundant agroindustrial residues into advanced materials, renewable energy carriers, and high-value-added bioproducts. This chapter provides a comprehensive analysis of the chemical composition, conversion pathways, and multifunctional applications of citrus residues within the framework of the circular bioeconomy. The intrinsic lignocellulosic matrix—rich in cellulose, hemicellulose, lignin, and pectin—serves as a renewable platform for the synthesis of nanocellulose, biochar, and functional carbon materials. Through thermochemical, biotechnological, and green-synthesis routes, bioactive constituents such as flavonoids, limonoids, and essential oils are recovered and transformed into bioadsorbents, photocatalysts, biofertilizers, and biopolymers. The chapter further examines the integration of citrus biorefineries in environmental remediation, energy storage, and biomedical systems, addressing critical challenges of process scalability, environmental safety, and life-cycle optimization. Future perspectives emphasize the convergence of green chemistry, digital technologies, and circular industrial models as the foundation for next-generation sustainable materials. Altogether, this multidisciplinary synthesis positions citrus residues as strategic precursors for eco-efficient and socially responsible innovations in the emerging green materials economy.