Nanoparticles and nanosheets of a high-performance aromatic polyimide can be synthesized via a simple monomer adsorption/polycondensation process inside the pores of specially selected mesoporous silica monoliths. The morphology of the pores directly controls the shape of the resulting polymer nanostructures. About perfect replication was found for both silicas under consideration: one a mesoporous silica with spherical pores with a diameter of 13 nm and another with a lamellar pore morphology with ca. 2 nm thickness. The resulting polymer structures were analyzed by TEM, SFM, thermogravimetric analysis, IR spectroscopy, and X-ray scattering, and the generation of a pure nanocrystalline polyimide was verified.