In this paper, we propose a compact and wideband electromagnetic bandgap (EBG) structure using a defected ground structure (DGS) to significantly enhance the wideband suppression of power/ground noise coupling in multilayer packages and printed circuit boards. The proposed EBG structure is implemented simply by adding a rectangular-shaped DGS which is etched periodically onto the ground plane without changing any other geometrical parameter from a mushroom-type EBG structure. The DGS effects on the fL and fU are thoroughly analyzed using the dispersion characteristics. We experimentally verified that the proposed EBG structure achieved the wideband power/ground noise suppression (below 40 dB) between 2.5 and 16.2 GHz. In addition, we demonstrated the considerable reduction in fL from 3.4 to 2.5GHz and a significant increase in fU from 9.1 to 16.2 GHz when compared with the mushroom-type EBG structure.