The integration of plasmonic nanoparticles (NPs), primarily gold (Au) and silver (Ag), onto portable, flexible, biodegradable, and low-cost cellulose paper substrates has attracted significant attention for developing sensing devices. These substrates, characterized by their porous structure, recyclability, flexibility, fluid transport capabilities, and ease of surface chemical modification, are ideal substrates to create innovative paper-based analytical devices (PADs). These platforms combine the advantages of colorimetric plasmonic sensing with the unique properties of paper, resulting in sensitive, cost-effective, and flexible analytical tools for detecting various targets, including biomarkers, biological compounds, pathogens, drugs, heavy metals, and gases. The straightforward fabrication process of PADs enables the production of multi-array devices, microfluidic circuits, and lateral flow assays (LFAs), leading to novel and alternative point-of-need (PON), and point-of-care (POC) applications. Additionally, integrating PADs with smartphones and wearable devices enables reagent-free, cost-effective, and portable colorimetric sensing of minimal sample volumes. This chapter presents the current progress using plasmon-based PADs, focusing on fabrication methods, operational principles, signal detection and amplification, and their diverse applications, including healthcare, environmental monitoring, and food safety.