In this paper, we propose a vertical stepped impedance electromagnetic bandgap (VSI-EBG) structure with a stopband enhancement and a size reduction for a wideband suppression of simultaneous switching noise (SSN) coupling in multilayer printed circuit boards (PCBs). The proposed VSI-EBG structure forms the stepped impedance EBG structure of power planes, which is implemented with a vertical branch, high-impedance (hi-Z) and low-impedance (low-Z) metal patches on different layers. Test vehicles are fabricated using a multilayer PCB process to verify the proposed VSI-EBG structure. Through experimental measurements, we verified the enhanced suppression of SSN coupling (below-40 dB) between 650 MHz and 20 GHz. In addition, we demonstrated that fL is reduced from 2.4 GHz to 650 MHz compared to the previous EBG structure, which allows an approximately 86% size reduction.