We herein report the preparation and crystallization behavior of polylactide (PLA) nanocomposites reinforced with polyhedral oligomeric silsesquioxane-modified montmorillonite (POSS-MMT), which is prepared by exchanging sodium cations of pristine sodium montmorillonite (Na-MMT) with protonated aminopropylisobutyl polyhedral oligomeric silsesquioxane (POSS-${NH_3}^+$). PLA nanocomposites with 1-10 wt% POSS-MMT contents are manufactured via melt-compounding, and their structures and melt-crystallization behavior are investigated. It is characterized that POSS-MMT nanoparticles in the nanocomposites have an exfoliated structure of MMT silicates with POSS-${NH_3}^+$ and partial POSS-$NH_2$ crystals. DSC cooling thermograms suggest that the overall melt-crystallization rates of the nanocomposite with only 3 wt% POSS-MMT are remarkably enhanced in comparison with the neat PLA. From the isothermal crystallization analysis based on the Avrami model, the overall melt-crystallization of PLA/POSS-MMT nanocomposites is found to be dominated by the heterogeneous nucleation and three-dimensional spherulite growth. Isothermal melt-crystallization experiments using a polarized optical microscope show that the spherulite nucleation density of PLA/POSS-MMT nanocomposites is much higher than that of the neat PLA, whereas the spherulite growth rates of all the nanocomposites are almost identical with the rate of the neat PLA. It is concluded that the highly enhanced melt-crystallization rates of PLA/POSS-MMT nanocomposites stem from the dominant nucleation effect of POSS-MMT nanoparticles for PLA crystals.