Mesalazine (5-aminosalicylic acid, 5-ASA), a currently used drug for anti-inflammatory bowel disease, is easily oxidized by HOCl, a strong oxidant generated in gut inflammation, to produce electrophilic quinones. We investigated whether this chemical feature has an implication in the anti-inflammatory pharmacology of 5-ASA. Human colon carcinoma HCT116 cells were treated with HOCl-reacted 5-ASA. Oxidized 5-ASA activated Nrf2 while 5-ASA itself was not effective. Activation of Nrf2 led to induction of hemeoxygenase (OH)-1, an anti-inflammatory enzyme. Western blot analysis of Keap1, a cytosolic repressor of Nrf2, following precipitation of biotin-labeled proteins in cell lysates treated with biotin-tagged 5-ASA, revealed a much greater amount of Keap1 when biotin-tagged 5-ASA was oxidized with HOCl. Precipitation of Keap1 was attenuated markedly by pretreatment with oxidized 5-ASA or a sulfhydryl donor. In addition, treatment with oxidized 5-ASA in cell lysates reduced the Keap1 amount that coimmunoprecipitated with Nrf2. In parallel, rectal administration of 5-ASA increased the level of HO-1 and nuclear Nrf2 in the inflamed colonic tissues, but not in normal colonic tissues. Moreover, oral gavage of sulfasalazine, a colon-specific prodrug of 5-ASA currently used clinically, activated the Nrf2-HO-1 pathway in the colonic tissues where inflammation was in progress, which was not observed when inflammation subsided. Collectively, our data suggest that Nrf2-HO-1 pathway is involved in the anti-inflammatory pharmacology of 5-ASA, which was likely being exerted exclusively in the inflammatory state.