Abstract: Colitis-associated colorectal cancer (CAC) is a major contributor to cancer-related mortality worldwide. Titanium dioxide (TiO₂, E171), a widely used food additive, has been insufficiently studied regarding its effects on macrophages within colon tumors during CAC development. In this study, CAC mouse models were used to investigate the biological impact of dietary E171 on macrophages in vivo, while lipopolysaccharide (LPS)-stimulated RAW264.7 macrophage cell lines were employed to elucidate the underlying mechanisms in vitro. We found that dietary E171 intake accelerated CAC development, exacerbated inflammatory responses and oxidative stress, and upregulated CAC-associated genes, including S100a8, S100a9, Lcn2, S100a11, Cxcl2, and interleukin-1α (Il-1α). E171 also increased the expression of S100A8, S100A9, NOD-like receptor family pyrin domain-containing 3 (NLRP3), and gasdermin-D N-terminal (GSDMD-N) in macrophages within colon tumors. In inflammatory macrophages, E171 exposure enhanced cell viability, increased reactive oxygen species (ROS) levels, and elevated the expression and secretion of S100A8 and S100A9, consistent with in vivo histological observations. Furthermore, E171-induced secretion of S100A8 and S100A9 in macrophages was suppressed by specific inhibitors, including N-acetylcysteine (NAC, ROS inhibitor), MCC950 (NLRP3 inhibitor), Z-YVAD-FMK (caspase 1 inhibitor), disulfiram (GSDMD inhibitor), and transfection of NLRP3 small interfering ribonucleic acid (siRNA). These results indicate that dietary E171 promotes CAC development by activating macrophages, with S100A8 and S100A9 serving as key mediators, and the NLRP3/caspase 1/GSDMD pathway acting as a critical mechanism.