Flaxseed (FS) is a dietary oilseed containing a variety of anti-inflammatory bioactives including fermentable fiber, phenolic compounds (lignans), and the n3-polyunsaturated fatty acid (n3-PUFA) α-linolenic acid. The objective of this study was to determine the effects of FS, and its n3-PUFA-rich kernel or lignan- and soluble fibre-rich hull, on colitis severity in a mouse model of acute colonic inflammation. C57Bl/6 male mice were fed basal diet (NEG), or basal diet supplemented with 10% FS, 6% kernel, or 4% hull for 3 weeks prior to and during colitis induction via 5 day dextran sodium sulfate (DSS; 2% w/v) in their drinking water (n=12/group). Although consumption of all FS-based diets increased anti-inflammatory metabolites (hepatic n3-PUFAs, serum mammalian lignans, and cecal short chain fatty acids (SCFAs)), this was not associated with anti-inflammatory effects in DSS-exposed mice. In fact, dietary FS exacerbated DSS-induced acute colitis as indicated by a heightened disease activity index and increased colonic injury and inflammatory biomarkers (histological damage, apoptosis, myeloperoxidase, inflammatory cytokines (IL-6 and IL-1β), and NF-B signalling-related genes (Nfkb1, Ccl5, Bcl2a1a, Egfr, Relb, Birc3 and Atf1)). Additionally, the adverse effect of FS diet was extended systemically, as serum cytokines (IL-6, IFN-, and IL-1β) and hepatic cholesterol levels were increased. The adverse effects of FS were not associated with alterations in fecal microbial load or systemic bacterial translocation (endotoxemia). Collectively, this study demonstrates that although consumption of a 10% FS diet enhanced the levels of n3-PUFAs, SCFAs, and lignans in mice, it exacerbated DSS-induced colonic injury and inflammation.