Phenol degradation with TiO2 photocatalyst under UV light is known to be an effective method. Under solar radiation, however, this approach does not receive adequate photons for catalyst activation, as the solar spectrum comprises mostly visible light (46%). In this study, we applied the dye-sensitization technique to prepare visible-light-active catalyst and used it under visible solar light generated from a solar simulator with a UV cutoff filter (λ > 420 nm) for phenol degradation. Eosin Y dye was used as a sensitizer for the TiO2 catalyst with a very low level of platinum as a cocatalyst. Triethanolamine was used as a sacrificial electron donor. Parametric studies were performed for the catalyst loading, initial triethnolamine concentration, initial phenol concentration, platinum content on TiO2, solution pH, and visible light intensity. About 93% degradation of 40 ppm phenol solution was achieved within 90 min using Eosin Y–TiO2/Pt photocatalyst under optimum conditions (pH 7.0, catalyst loading of 0.8 g/L, triethnolamine concentration of 0.2 M, 0.5% Pt loading on TiO2, visible solar light intensity of 100 mW/cm2). The kinetic rate constant and adsorption equilibrium constant were determined, and a Langmuir–Hinshelwood-type equation was proposed to describe phenol degradation on TiO2 at different visible light intensities. The model equation was found to predict the experimental results quite well.