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Although current medications for ulcerative colitis (UC) are effective to some extent, there are still some limitation of their use due to the non-specific distribution, drug metabolism in the gastrointestinal tract, and severe adverse effects. In our previous studies, we developed oral redox nanoparticles (RNPO) that specifically accumulated and scavenged overproduced reactive oxygen species (ROS) in an inflamed colon. However, the mechanism leading to specific accumulation of RNPO in an inflamed colon is still unclear. In this study, we investigated the cellular uptake of RNPO into ROS-treated epithelial colonic cells in vitro, and compared to the untreated cells, found a significantly increased uptake in ROS-treated cells. In vivo, we discovered that orally administered RNPO were not internalized into the cells of a normal colon. A significant amount of disintegrated RNPO was detected in the cells of an inflamed colon of dextran sodium sulfate (DSS)-induced colitis mice, resulting in scavenging of ROS and suppression of inflammation with low adverse effects. Furthermore, we confirmed a significant reduction of disease activity and a robust dose response efficacy following RNPO treatment in acute DSS-induced colitis mice, outperforming the positive control 5-aminosalicylic acid. Oral administration of RNPO is a promising approach to develop a new therapy for UC disease.