5-Fluorouracil (5-FU) is a commonly used chemotherapeutic agent for various cancers. However, the drug resistance developed by tumor cells hinders the therapeutic effect. Konjac glucomannan (KGM) is indicated to sensitize 5-FU-resistant hepatocellular carcinoma (HCC) cells to 5-FU. In our study, we found that KGM or 5-FU treatment alone did not affect the malignant cell behaviors and endoplasmic reticulum (ER) stress of 5-FU-resistant HCC cells or HepG2/5-FU and Bel-7402/5-FU cells, while cotreatment with KGM and 5-FU significantly facilitated HCC cell apoptosis and ER stress and suppressed cell proliferation potential and migration abilities. Moreover, we explored the underlying mechanism by which KGM induces 5-FU cytotoxicity in HCC cells. We found that Toll-like receptor 4 (TLR4) was downregulated in KGM- and 5-FU-treated HCC cells. TLR4 overexpression reversed the KGM and 5-FU cotreatment-induced inhibition of the malignant behaviors of 5-FU-resistant HCC cells. Furthermore, KGM enhanced 5-FU-induced ER stress by inhibiting TLR4 to activate PERK/ATF4/CHOP signaling. Xenograft mouse models were established using HepG2/5-FU cells, and KGM was demonstrated to reverse 5-FU resistance in HCC tumorsin vivoby suppressing TLR4 to enhance ER stress and activate PERK/ATF4/CHOP signaling. In conclusion, KGM combined with 5-FU treatment significantly promoted apoptosis and reduced cell proliferation, migration and ER stress in 5-FU-resistant HCC cells compared with KGM or 5-FU treatment alone by downregulating TLR4 to activate PERK/ATF4/CHOP signaling.