Metformin hydrochloride is an extensively used antidiabetic drug that according to the results reported here is able to spontaneously intercalate layered silicates like the montmorillonite clay mineral following an ion-exchange mechanism. The adsorption isotherm from water solutions shows a great affinity of metformin towards the clay mineral, which can retain about thrice the exchange capacity of the clay. The adsorbed excess was easily removed by washing with water, leading to an intercalation compound that contains 93 meq of metformin per 100 g of montmorillonite, matching the CEC value of this clay. The intercalated metformin is arranged in the interlayer space as a monolayer of monoprotonated molecules, which remain strongly entrapped within the solid. These new hybrid materials were characterized by elemental chemical analysis, XRD, FTIR, TG-DTA, and NMR. We preliminary evaluated the use of the metformin-montmorillonite intercalation compound as a drug delivery system, determining the liberation kinetics of metformin at diverse pH values that mimic the gastrointestinal tract. Although the release rate was not totally slowed down, the system seems promising in view of further optimization for drug delivery applications.