The dramatically increasing incidence and prevalence of inflammatory bowel disease (IBD) are reportedly related to a Western diet, which is characterized by high sugar consumption. Dietary simple sugars aggravate IBD in animal models. However, the mechanisms underlying this effect remain unclear. Given that high-fructose corn syrup (HFCS) is a major added sugar in food and beverages, we focus on HFCS and investigated the effects of HFCS on a dextran sulfate sodium (DSS)-induced murine colitis model and in RAW264.7 macrophages. Our data demonstrate that short-term consumption of HFCS aggravates colitis and upregulates the proportion of macrophages in IBD mice but not in healthy mice. We find that HFCS promotes proinflammatory cytokine production through reactive oxygen species (ROS)-mediated nuclear factor-κB (NF-κB) signaling in RAW264.7 macrophages. Furthermore, N-acetylcysteine (NAC), an ROS scavenger, alleviates HFCS-aggravated colitis in mice and inhibits the ROS-mediated NF-κB signaling pathway in RAW264.7 macrophages. Our work unveils the important role of macrophages in HFCS-induced exacerbation of colitis and reveals the mechanism of how HFCS immunologically aggravates IBD.Keywords: High fructose corn syrup; Inflammatory bowel diseases; Macrophage; Nuclear factor-κB; Reactive-oxygen-species.