A new rhodamine B derivative (RDB) was synthesized and utilized for the colorimetric detection of copper ions (Cu). This chemosensor utilized a paper strip as a support and a smartphone as a detector for on-site quantitative detection of Cu in water samples. Silica nanoparticles (SiNPs) were investigated as the modifier nanoparticles to achieve uniform color on the paper strip and showed a color response 1.9-fold higher than the one without SiNPs. The RDB chemosensor-based paper strip provided high selectivity toward Cu with a detection limit of 0.7 mg/L, and the working concentrations for Cu ranged from 1 to 17 mg/L. Parallel analyses of eight drinking water samples were conducted by inductively coupled plasma optical emission spectroscopy. The results were in good agreement, indicating the practical reliability of the established method with a short assay time and high selectivity. These indicate its great potential for on-site detection of Cu.