Lightweight and robust polybenzoxazine foams were prepared using a simple sol-gel method based on a commercially available benzoxazine monomer, BA-a, with hexamethylenediamine as a curing agent and methanol/chloroform as the solvent. By varying the initial concentrations of organic solutions, polybenzoxazine foams with densities of 0.2815 g/cm(3) (F-20), 0.4798 g/cm(3) (F-30), and 0.6731 g/cm(3) (F-40) were prepared. Scanning electron microscopy (SEM) images revealed that all foams possessed open-cell structures, and the morphology of the pore walls can be readily tailored by initial concentrations. As a consequence of the porous structure, the polybenzoxazine foams showed a low dielectric constant (1.47 at 1 MHz for F-20), low dielectric loss (0.003 at 1 MHz for F-20), low thermal conductivity (0.0604 W/(m K) for F-20), and low coefficient of linear expansion value (55.1 ppm/degrees C for F-20). Moreover, foam F-40 showed a favorable T-g of 191.8 degrees C, char yield of 46.1%, and compressive strength of 71.74 MPa. The outstanding comprehensive performances make the polybenzoxazine foams prepared using this methodology good candidates as lightweight, robust, and thermally stable matrices for many functional applications.