Konjac glucomannan (KGM) is a dietary fiber found in Amophophallus konjac. This fiber is fermentable based on human and animal trials, but short-chain fatty acid (SCFA) production profiles are unknown. The aim of this study is to characterize the digestibility and fermentability in vitro of two preparations of KGM, to better understand how KGM improves human health. Konnyaku (yam cake made of A. konjac), isolated KGM, inulin, and guar gum were subjected to in vitro digestion and in vitro fermentation. Fermentation samples were removed at 0, 4, 8, 12, and 24 hours for gas volume, pH, and SCFA measurements. Acetate, propionate, and butyrate were measured with gas chromatography. Results of the in vitro digestion confirm that KGM and konnyaku are resistant to degradation by digestive enzymes. Gas production in fermentation vessels containing konnyaku and KGM was lower than for inulin from 8 to 24 hours. Both samples produced SCFA concentrations similar to guar gum, which favored acetate and propionate over butyrate production. This study is the first to characterize SCFA production by KGM in its isolated form and in food form. Fermentation patterns presented in this study may provide a mechanism for the previously published health benefit of konnyaku and KGM.