<p>This study aimed to evaluate the effects of microwave treatment on the physicochemical properties of konjac glucomannan (KGM) and the prebiotic potential of KGM fractions with varying molecular weights (Mw). After microwave treatment, KGM exhibited a significant reduction in Mw which resulted in dramatic structural alterations including a transition to rod-like chain conformation and amorphous crystallinity, and fragmented surface morphology. The in vitro fermentation assay revealed that microwave-treated KGM fractions can be more effectively metabolized by gut microbiota, and the lower the molecular weight, the superior the fermentability. Strikingly, both total short-chain fatty acids (SCFA) and valeric acid in lower-Mw KGM fractions showed significantly higher concentrations than those in inulin. Microbiota diversity and composition analysis revealed that consumption of microwave-treated KGM significantly increased the relative abundance of beneficial gut microbiota, especially <i>Firmicutes</i>, <i>Actinobacteria</i>, and <i>Lactobacillus</i>, indicating enhanced prebiotic activity.</p> Graphical Abstract <p></p>