Biosensors are analytical devices with a wide range of uses in various fields such as food, military, environmental monitoring, and clinical diagnostics. Similarly, polymers and their composites have sparked a lot of interest in biosensing because of their properties, including compatibility with biological molecules, efficient electron transfer during biochemical reactions, bioreagent loading, and biomolecule immobilization. Different nanoparticles such as carbon nanotubes, graphenes, gold nanoparticles, etc., have been efficiently integrated into the polymer matrix to improve performance features such as rapid response, high selectivity, improved sensitivity, long-term stability, and lower detection limit. Polymers in combination with nanomaterials provide a three-dimensional matrix that preserves biomolecule activity and provides an excellent platform for immobilization due to their good durability, porosity, selectivity, high electron transfer rate, biocompatibility, and better signal transduction. This chapter focuses on polymer nanocomposites for biosensing applications in a wide range of biological species.