Polypropylene (PP) is one of the most widely used commodity polymer. However, it has a low upper service temperature. Addressing this challenge, we introduced reactive blending of PP, maleic anhydride-grafted PP (PP-g-MA) and (3-aminopropyl)triethoxysilane (3APTES) in the presence of nanoclay, which results in a highly nucleated PP composite with improved dimensional stability. At the processing temperature, the reaction between maleic anhydride, which is already grafted on the PP chain, and 3APTES forms N-substituted maleimide-grafted PP. The results indicated that the introduction of branched structures on PP chains increased the chain bulkiness and led to the formation of stable nuclei that initiate crystallization at high temperatures. By offering nucleating sites, the dispersed nanoclay particles facilitated the nucleation process to a greater extent. Compared with the neat polymer matrix, the crystallization temperature of the composite was 15.5 °C higher with moderate improvements in the heat distortion and Vicat softening temperatures. This is most likely the first report where such an improvement in crystallization temperature has been achieved for isotactic PP homopolymers. In summary, reactive processing of the PP/nanoclay composite in the presence of PP-g-MA and 3APTES has potential for the development of high-performance PP-based advanced materials.