The aim of this study was to investigate the effects of foaming on the electrical properties of carbon nanotube (CNT)-reinforced polystyrene (PS). A pseudo-three-dimensional (3D) model based on random walk simulation was developed for predicting the electrical properties of CNT nanocomposites. The electromagnetic interference shielding effectiveness (EMI SE) of foamed PS/CNT composites was also studied through a network analyzer, measuring the EMI SE of specimens through reflection and absorption mechanisms. Six types of nanocomposites, including foamed and nonfoamed PS/CNT composites with a CNT loading of 2.1 vol% and different void contents, were manufactured using compression molding and leaching techniques. To realize the effects of foaming on the electrical conductivity of PS/CNT composites, electrochemical impedance spectroscopy analyses were carried out and compared with the pseudo-3D model. We also found that foaming via the leaching method improved the EMI shielding properties of the composites up to 24%.