© 2021, The Polymer Society, Taipei.Bimodal poly(methyl methacrylate) (PMMA) foams are produced with a new method combining syntactic foam production and supercritical CO2 (scCO2) foaming. Bimodal pore structure offers several potential advantages to foam properties, such as reduced mass density and excellent thermal and sound insulation properties. To produce the foams, first PMMA composites are produced with hollow glass microspheres (HGMs) and octatrimethylsiloxy polyhedral oligomeric silsesquioxane (POSS) nanoparticles using a twin screw extruder. The composite syntactic foams are then processed with scCO2 to form bimodal pore structured PMMA-HGM-POSS composite foams. In the foam matrix, large pores form due to the CO2 diffusion in the matrix towards the voids between the matrix and HGM surfaces, and small pores are formed at the nucleation cites. Highly CO2-philic POSS nanoparticles contribute to scCO2 foaming as cell nucleators enhancing the foaming performance of the syntactic composites. The effects of process pressure, time, and concentrations of HGMs and POSS nanoparticles on the foam morphology are studied. The highest decrease achieved in the foam density compared to that of the bulk polymer is 69% with only 12% decrease in the hardness.