A novel rhodamine B derivative (Rh-2) was synthesized. When used as a selective colorimetric sensor for Cu2+, a visible change from colorless to pink was observed. UV-visible and fluorescence spectroscopy demonstrated linear relationships between the concentrations of Cu2+ and Rh-2 in ethanol/water mixtures. Since naked eye detection is inexpensive and more convenient than spectroscopy, test strips were fabricated by immersing bacterial cellulose (BC) in Rh-2 and applying heat treatment. The resulting solid sensor was able to detect Cu2+ in water, visibly changing from colorless to pink. A linear correlation was found between color strength and Cu2+ concentration, when plotted on a log-log scale. No leaching of Rh-2 from BC was observed due to the strong affinity between the two. The BC had a more uniform nanofiber size and larger surface-area-to-volume ratio than the commonly-used cellulosic paper, making it a promising material for use in colorimetric sensors.