<p>Nanomaterial-enhanced composite membranes are increasingly used to overcome the limitations of conventional polymeric membranes in desalination and wastewater treatment. Incorporating nanomaterials such as metal oxides, carbon-based structures, MXenes, and porous frameworks can improve water permeability, contaminant rejection, mechanical strength, and fouling resistance. This review summarizes advances in nanocomposite membrane development and explains how different nanomaterial structures—classified as zero-dimensional, one-dimensional, two-dimensional, and three-dimensional—interact with polymer matrices to enhance membrane properties. Applications across reverse osmosis, nanofiltration, ultrafiltration, membrane distillation, and wastewater pollutant removal are discussed, including examples from both research studies and commercial thin-film nanocomposite (TFN) membranes. Key practical challenges such as nanomaterial agglomeration, long-term stability, large-scale manufacturing, and environmental safety are also examined. Recent strategies to improve dispersion, reduce nanoparticle leaching, and adopt more sustainable fabrication methods are reviewed. Finally, potential future directions are highlighted, focusing on multifunctional hybrid fillers, improved structural design, and broader industrial implementation. Overall, this review shows that nanocomposite membranes offer strong potential to support more efficient, durable, and environmentally sustainable water-treatment systems if current challenges can be successfully addressed.</p>