A naphthalene-based bichromophoric fluorescent sensor 2,2′-[oxy-bis(2-oxatetramethyleneoxy)]-bis[N-(2-naphthyl)-benzamide)] (1) was synthesized and characterized. Fluorescence decay for 1 in alcoholic solvents in the region of 415–460 nm revealed bi-exponential behavior. The faster component of the decay can be attributed to the formation of dimers. Above 480 nm, besides the dimer, there is also a little excimer formation and this excimer emits at longer wavelengths than the dimer. The observation of the change of the fluorescence emission spectra upon addition of water in EtOH–water mixtures is in line with the formation of water-bridged complexes preventing excimer formation. The sensor shows an increase in fluorescence intensity upon increasing Mg²⁺ or Ca²⁺ concentration in EtOH because the formation of the excimer can be hindered upon complexation with Mg²⁺ or Ca²⁺ ions. Because of the competition between hydrated metal ions and the water-bridged complex, spectral changes by complexation with Mg²⁺ or Ca²⁺ in EtOH–H₂O (9 : 1 v/v) are quite different from those in neat ethanol. The ground-state dissociation constant K_d estimated for the complex with Mg²⁺ or Ca²⁺ was found to be around 2.0 mM in EtOH–H₂O (9 : 1 v/v), which makes it suitable for the measurement of the concentrations of these ions in physiologically relevant concentration ranges.