Nanocellulose is a versatile green material derived from natural sources such as plants, bacteria, and biomass. It is highly renewable, biodegradable, and possesses excellent mechanical strength, biocompatibility, and tunable self-assembly properties. This review discusses the synthesis, surface modification, and applications of nanocellulose, highlighting its potential in various fields including materials science, biomedicine, energy, and environmental remediation. Nanocellulose can be obtained from biomass through mechanical, chemical, or enzymatic treatments, and its properties can be tailored through surface modification techniques such as phosphorylation, esterification, and silylation. It is used to fabricate a wide range of nanomaterials and nanocomposites, including those based on polymers, metals, metal oxides, and carbon. Nanocellulose is particularly useful in reinforcing composite materials, providing mechanical strength, and offering functional properties such as thermal stability, flame retardancy, and high surface area. Its applications include functional paper, optoelectronics, antibacterial coatings, packaging, tissue scaffolds, drug delivery, biosensors, energy storage, catalysis, and environmental remediation. Phosphorylated nanocellulose is especially promising for applications such as bone scaffolds, adsorbents, and flame retardants. The review also covers the different types of nanocellulose, including cellulose nanocrystals (CNCs), nanofibrillated cellulose (NFC), and bacterial nanocellulose (BNC), and their unique properties and applications. The review emphasizes the importance of nanocellulose as a sustainable and eco-friendly material for future technological and industrial applications.Nanocellulose is a versatile green material derived from natural sources such as plants, bacteria, and biomass. It is highly renewable, biodegradable, and possesses excellent mechanical strength, biocompatibility, and tunable self-assembly properties. This review discusses the synthesis, surface modification, and applications of nanocellulose, highlighting its potential in various fields including materials science, biomedicine, energy, and environmental remediation. Nanocellulose can be obtained from biomass through mechanical, chemical, or enzymatic treatments, and its properties can be tailored through surface modification techniques such as phosphorylation, esterification, and silylation. It is used to fabricate a wide range of nanomaterials and nanocomposites, including those based on polymers, metals, metal oxides, and carbon. Nanocellulose is particularly useful in reinforcing composite materials, providing mechanical strength, and offering functional properties such as thermal stability, flame retardancy, and high surface area. Its applications include functional paper, optoelectronics, antibacterial coatings, packaging, tissue scaffolds, drug delivery, biosensors, energy storage, catalysis, and environmental remediation. Phosphorylated nanocellulose is especially promising for applications such as bone scaffolds, adsorbents, and flame retardants. The review also covers the different types of nanocellulose, including cellulose nanocrystals (CNCs), nanofibrillated cellulose (NFC), and bacterial nanocellulose (BNC), and their unique properties and applications. The review emphasizes the importance of nanocellulose as a sustainable and eco-friendly material for future technological and industrial applications.