Organic molecular sensors have the advantage of being used through easy, fast, economical and reliable optical methods for detecting toxic metal ions in the environment. In this work, we present a simple but highly specific organic ligand 5-Chloro-2-[(1E,2E)-3-(4-(dimethylamino)phenyl)allylidene)amino)]phenol (S1) that works as a colorimetric sensor of copper ions in aqueous solutions. Binding interaction between S1 and various metal ions was established by UV–Vis spectroscopic measurements showing favorable coordination toward Cu²⁺ and practically no interference with the presence of other metal ions, i.e., Cd²⁺, Co²⁺, Cr²⁺, Fe³⁺, Mg²⁺, Ni²⁺, Hg²⁺, Pb²⁺, Mn²⁺, and Zn²⁺. S1 exhibited binding-induced color changes from yellow to pink with a detection limit of 1.25 × 10⁻⁷ M measured by spectroscopic methods, while colorimetric changes could be observed at naked eye for concentrations as low as 2 × 10⁻⁶ M. Furthermore, we also demonstrated that S1 can be useful for sensing copper ions through the use of surface plasmon resonance (SPR) technique. This technique allowed detecting the presence of Cu²⁺ in aqueous solutions at the concentration level of ca 1.5 × 10⁻⁶ M due to changes in the refractive index.