We propose a simple model describing the retention of selenite, Se(IV), by the MX-80 bentonite in a synthetic groundwater (SGW) in dispersed and compacted states. The model was calibrated on a pure montmorillonite from data obtained for 4 < pH < 10 and total selenite concentrations between 10−7 and 5 × 10−3 mol/L. Furthermore, the matrix solution covers a wide range of conditions regarding the ionic strength and the nature of the background electrolyte. Three selenite surface species had to be considered. Below pH 5, sorption is governed by a ligand exchange reaction with H2SeO3. Between pH 5 and 7, the experimental data are well described considering the formation of a surface complex implying HSeO3−. Finally, at pH above 7, we propose ternary surface complexes involving Ca2+ and Mg2+. The model, which we consider as operational, appears in agreement with spectroscopic data available in the literature and predicts surprisingly well selenite sorption on MX-80 bentonite even in the compacted state at a dry density of 1100 kg/m3. Based on the model, the solid phase montmorillonite is responsible for selenite retention. Minor solid phases containing iron (pyrite, hematite) had not to be considered in the modelling. Interestingly, calcite (an important phase in MX-80 bentonite) has an indirect effect via the release of calcium into solution and its subsequent contribution to Se(IV) sorption through a ternary surface complex.