Identification of biomarkers is essential for prognosis, treatment monitoring, and early disease detection. Because of their special physicochemical characteristics, including high surface-to-volume ratios, adjustable optical and electrical characteristics, and superior biocompatibility, nanoparticles have become effective instruments in biosensing applications. Using a variety of biosensing techniques, such as optical nano-biosensors, electrochemical methods, nanoprobes, and immunoassay sensors, this chapter investigates the function of nanoparticles in biomarker detection. Highly sensitive and selective biomarker detection is made possible by optical nano-biosensors, such as fluorescence-based, surface plasmon resonance (SPR), and surface-enhanced Raman spectroscopy (SERS) sensors. Electrochemical methods, such as electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV), provide quick, affordable diagnostic platforms with superior detection limits. For the detection of biomarkers, nanoprobes functionalized with biomolecules such as DNA, enzymes, or antibodies offer improved selectivity and signal amplification. Furthermore, point-of-care diagnostics are made more accurate by immunoassay sensors, such as lateral flow assays and nano-enabled ELISA. This chapter compares several approaches in terms of sensitivity, specificity, and practicality to highlight the benefits and difficulties of nanoparticle-based biosensors. Lastly, to demonstrate how nanoparticle-based biosensors have the potential to transform clinical diagnostics, new developments and future directions are explored, including AI integration and multi-modal sensing platforms.