<p>Understanding the water vapor diffusion characteristics of bamboo is crucial for optimizing the manufacturing of bamboo-based products. Its radial structure, composed of distinct anatomical regions—such as the bamboo outer layer (BOL), inner layer (BIL), pith ring (BPR), and membrane (BM)—results in variations in chemical composition and contributes to complex moisture diffusion behavior. To explore how these regions contribute to moisture transport, three types of bamboo samples were prepared: intact (BOL/BM), BM removed (BOL/BPR), and both BM and BPR removed (BOL/BIL). Water vapor diffusion was assessed using the wet cup method (humidity levels of 85% and 0%). The work measured diffusion in both the outer-to-inner (BM-BOL, BPR-BOL, BIL-BOL) and inner-to-outer (BOL-BM, BOL-BPR, BOL-BIL) directions. The results indicated that the moisture diffusion was significantly more efficient in the inner-to-outer direction compared to the outer-to-inner direction. BOL-BIL showed the lowest water vapor resistance factor (<i>µ</i> = 62.73 ± 1.55), attributed to the exposure of parenchyma cells and vessels following BPR removal. Conversely, BOL-BPR showed the highest resistance (<i>µ</i> = 108.96 ± 4.93) due to the high lignin content and thick-walled cell structure of BPR. The BM’s layered and porous architecture, formed by collapsed pith cells, facilitated efficient moisture diffusion, endowing it with optimal hygroscopic properties. However, BPR’s inhibitory effect increased resistance in BOL-BM (<i>µ</i> = 81.17 ± 2.43). This study elucidates the distinct roles of BPR and BM in water vapor diffusion within bamboo, enhancing the understanding of its internal moisture diffusion mechanisms and providing a foundation for the development of moisture-resistant bamboo materials.</p>