Herein, we present a novel spectroelectrochemical fiber-optic sensor that combines electrochemistry, spectroscopy, and electrostatic adsorption in three modes of selectivity. The proposed sensor is simple and consists of a gold mesh cover on a multimode fiber optic that uses attenuated total reflection as the optical detection mode. The sensing is based on changes in the attenuation of the light that passes through the fiber-optic core accompanying the electrochemical oxidation-reduction of an analyte at the electrode. Methylene blue and ferrocyanide were used as model analytes to evaluate the performance of the proposed sensor. The optical transmission changes generated by electrochemical manipulation showed a good linear relationship with the concentration and the limits of detection (3σ) for methylene blue and ferrocyanide at 2.0 × 10(-7) and 1.6 × 10(-3) M, respectively. The sensor responses were successfully enhanced with an additional level of selectivity via an electrostatically adsorbed, self-assembled monolayer (SAM), which consisted of a silane coupling layer, a polyanion, and a polycation. The improvement observed in the sensitivity of a SAM-modified fiber-optic sensor was rather encouraging. The optimized sensor had detection limits (3σ) of 8.3 × 10(-9) M for methylene blue and 7.1 × 10(-4) M for ferrocyanide. The developed sensor was successfully applied to the detection of ferrocyanide in simulated nuclear waste.