The effect of the amount of nanofiller and type of organic modifier in organomodified montmorillonite (OMMT)/poly(methyl methacrylate) (PMMA) nanocomposites prepared by in situ bulk polymerization was investigated. Several commercial OMMTs under the tradename Cloisite were used. The structure and morphology of the prepared materials were investigated by means of wide angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analysis. It was found that the presence of OMMT enhances polymerization kinetics especially in the gel-effect region, while sodium MMT acts rather as a reaction retarder. The presence of the nanofiller increased the thermal stability of the nanocomposites, as measured by thermogravimetric analysis (TGA) and their glass transition temperature, measured by differential scanning calorimetry (DSC), as well as their average molecular weight measured by gel permeation chromatography (GPC). Tensile measurements using a dynamometer revealed an increase in the Young's modulus with the amount of the nanofiller accompanied by a decrease in the tensile strength and elongation at break. Overall, the best performance was found for the nanocomposites of PMMA with Cloisite 25A.