Geosynthetic Clay Liner (GCL) systems are used as efficient hydraulic barriers in landfills for the disposal of hazardous municipal wastes. Along with geotextiles, bentonite materials are chosen as one of the primary components of GCLs due to their high retention, adsorption, and swelling capacities. GCLs are manufactured using bentonites at a high total suction and hydrated through the uptake of liquid from the subsoil and the confined material as soon as they are installed. Bentonites may exhibit considerable volume change upon wetting. Depending on the confinement stress, the void ratio may significantly increase with a decrease in suction, particularly at higher degrees of saturation. To improve the hydraulic performance of GCLs, the swelling of the bentonites induced by the hydration should be limited when GCLs reach low suction values. The change in the hydrated void ratio is related to the pore structure evolution of the bentonites at different hydration levels. An improved understanding of the water retention and the void ratio evolution of bentonite materials during swelling is required, with particular attention given to the applications of GCL systems. The aim of this paper is to characterise the water retention and the swelling behaviour of granular bentonites for applications in GCLs. MX-80 granular bentonite, with an optimised grain size distribution, and Volclay GC-50 granular bentonite were selected, and their water retention behaviour was determined based on a new methodology. Reconstituted GCL specimens with MX-80 and Volclay GC-50 granular bentonites were tested with the same methodology to determine the water retention behaviour. An analysis of the water retention behaviour of the granular bentonites and reconstituted GCLs indicates that an adsorption mechanism controls the water retention behaviour for a wide range of total suction values. An analysis of the swelling potential of the granular bentonites indicates a significant increase in the hydrated void ratio for lower suction values. The increase in the void ratio is attributed to the modification of the smectite particles in the hydration path that results in new pore levels emerging in the bentonite structure. This change in the void ratio is expected to influence the hydraulic performance of the GCL in terms of diffusion and hydraulic conductivity.