The aim of this study was to clarify the morphologic and spatial relationships of the inferior bundle of the buccinator and the incisivus labii inferioris muscle (ILI) and their surrounding structures. The inferior bundle of the buccinator and the ILI were investigated in 40 hemifaces from Korean cadavers. The inferior bundle (or fourth band) of the buccinator muscle was observed in 14 (35%) of the 40 specimen sides. The ILI was found in 39 (97.5%) of the 40 specimen sides. The spatial relationships of the ILI with the buccinator muscle and the orbicularis oris muscle were classified into 4 categories based on the existence of the inferior bundle of the buccinator. These observations indicate that the lower portion of the buccinator including the third and fourth inferior bundles and the ILI could affect the alveolar bone of the mandible or occlusion during these muscular movements.