The cotton plant (Gossypium spp.) is widely cultivated for its fibre, yet it represents a significant source of diverse phytochemicals and industrially valuable by-products. This review summarises the major chemical constituents of cotton, including primary metabolites such as cellulose, proteins and fatty acids and secondary metabolites such as gossypol, terpenoids, flavonoids, phenolic acids, tannins and coumarins. Special emphasis is placed on the pharmacologically active substances such as gossypol and various terpenes, highlighting their roles in antimicrobial, anticancer and antioxidant activity. In addition to its primary fibre product, cotton generates a variety of by-products including cottonseed oil, meal, hulls and linters which serve applications in food, feed, pharmaceuticals and biotechnology. The review further examines the impact of genetic variation, transgenic modifications and environmental conditions on the plant's chemical profile. The synthesis of current knowledge demonstrates that cotton is not only a major fibre crop but also a valuable source of bioactive compounds with significant industrial and biomedical potential.