This chapter provides a detailed overview of coir fiber’s fundamental properties and applications, focusing on its integration into geopolymer composites. As a versatile natural fiber extracted from coconut husks, coir is distinguished by its high lignin content and biodegradability, which contribute to its robustness and durability. The chapter begins with an examination of coir physical and chemical properties, comparing untreated and chemically modified coir fibers derived from distinct coconut varieties. Key parameters such as fiber morphology, tensile strength, and thermal stability are analyzed to underscore coir suitability for composite materials. The latter section discusses the incorporation of treated coir fibers into geopolymer matrices, where they enhance mechanical properties, specifically flexural strength and durability. This enhancement is attributed to the interaction between the fiber surface and the geopolymer binder, which is discussed in detail. Findings demonstrate that specific chemical treatments on coir fibers improve their compatibility and adhesion within the geopolymer, expanding coir applications in sustainable engineering materials. This chapter acts as a stepping-stone for further research on coir fibers potential in composite technology, offering insights into its properties, treatment methodologies, and practical applications in eco-friendly composite materials.