<p>Biodegradable polysaccharide-based carbon quantum dots (CQDs) have garnered significant attention as biocompatible and multifunctional nanomaterials with promising applications in biomedical science. Their facile synthesis from renewable polysaccharides offers an eco-friendly and sustainable approach, reducing concerns regarding toxicity and environmental impact. These CQDs exhibit excellent photostability, tunable photoluminescence, and high-water solubility. In cancer diagnosis, functionalized CQDs enable precise tumor visualization and early detection by selectively binding to cancer biomarkers, including circulating tumor DNA and exosomes. Their potential extends to non-invasive liquid biopsy applications, enhancing cancer screening accuracy. Unique optical properties allow them to function as photosensitizers or photothermal agents, enabling targeted photodynamic and photothermal therapy. Despite their immense potential, challenges such as large-scale synthesis, reproducibility, and long-term cytotoxicity need to be addressed for their clinical translation. This review explores the synthesis, physicochemical characteristics, and biomedical applications of biodegradable polysaccharide-based CQDs, emphasizing their role in cancer diagnosis and therapy while highlighting future prospects and challenges in their clinical application.</p>