A cationic porphyrin, 5,10,15,20‐tetrakis(1‐methyl‐4‐pyridinio)porphyrin tetraiodide (TMPyP) was reported herein to exert selective, turn‐off fluorescence when complexed with Cu2+ ion. Although discernible spectroscopic changes of charged porphyrins with various metal ions were previously utilized as colorimetric sensors, the application of fluorescence quenching of charged porphyrins with Cu2+ ion has never been investigated in much detail before. Coupled with the ease of synthesis, the finding led to a construction of a simple paper‐based sensor for Cu2+ with good selectivity and sensitivity, with a limit of detection at 0.16 ppm, which is below the recommended amount in drinking water. Notably, this performance could be achieved with the need of only a handheld UV lamp and a smartphone or a compact camera, both of which are now considered to be highly accessible. Thanks to the simplicity of all involved processes, this study highlights a unique way in creating a new practical and economical sensor for Cu2+ ion.