Paraquat has been widely used in agriculture due to its efficient herbicidal activity. However, its remaining residues in the environment have caused eco-toxicity to live organisms and humans. Therefore, a portable and sensitive sensing system is required for field-monitoring paraquat towards eco-friendly usage. Toward this goal, citrate-capped silver nanoparticles (AgNPs) on a microfluidic paper analytic device (µPAD) were used to detect paraquat, accompanied by smartphone detection. Capillary action through paper fibers enabled the distribution of pre-loaded AgNPs across paper microfluidic channels and allowed rapid citrate-paraquat binding, which led to rapid, reproducible, and sensitive detection of paraquat. Full extents of color changes were recorded within 1 min using a smartphone camera, using only 2 µL samples. The assay's linear range was from 2.5 to 20 ppm, and the limit of detection was 2.3 ppm (8.94 µM) paraquat. Successful selectivity was also demonstrated using several different ions and herbicides, without substantial non-specific aggregation. Moreover, paraquat was further assayed with co-existing ions and field water samples, with satisfactory analytical recoveries of 95−117%, demonstrating its potential application towards on-site and field-ready assay of paraquat.