© 2021 Wiley Periodicals LLC.Highly CO2-philic nanoparticles, octatrimethylsiloxy polyhedral oligomeric silsesquioxanes (POSS) are used to increase the affinity of poly(methyl methacrylate) (PMMA) to CO2 in supercritical carbon dioxide (scCO2) foaming, thus to improve its foaming performance and the foam morphology. PMMA and PMMA-POSS composite foams were produced based on the two-factorial design, at the upper and lower experimental conditions of pressure, temperature, processing time, and venting rate. The foams of PMMA-5% POSS composites exhibited smaller average pore sizes and higher pore densities than neat PMMA and PMMA-0.5% POSS composites. The smallest average pore diameter (0.3 μm) and the highest pore density (6.33 × 1012 cm−3) were obtained with this composite processed at 35°C, 32 MPa, for 24 h and depressurized with fast-venting rate (0.4 MPa/s). ScCO2 processing decreased the density of the polymer by more than 50%.