: Chronic inflammation and Western-style diets elevate colorectal cancer (CRC) risk, particularly in individuals with colitis, a feature of inflammatory bowel disease (IBD). Diets rich in polyphenol-containing functional foods, such as cocoa, may reduce gut inflammation and modulate the gut microbiome. This study investigated the impact of cocoa polyphenol (CP) supplementation on inflammation and microbiome composition in mice with colitis, fed either a healthy or Western diet, before, during, and after the onset of disease. We hypothesized that CPs would attenuate inflammation and promote distinct shifts in the microbiome, especially in the context of a Western diet. : A 2 × 2 factorial design tested the effects of the basal diet (AIN93G vs. total Western diet [TWD]) and CP supplementation (2.6% / CocoaVia™ Cardio Health Powder). Inflammation was induced using the AOM/DSS model of colitis. : CP supplementation did not reduce the severity of colitis, as measured by disease activity index or histopathology. CPs did not alter gene expression in healthy tissue or suppress the colitis-associated pro-inflammatory transcriptional profile in either of the two diet groups. However, fecal microbiome composition shifted significantly with CPs before colitis induction, with persistent effects on several rare taxa during colitis and recovery. : CP supplementation did not mitigate inflammation or mucosal injury at the tissue level, nor did it affect the expression of immune-related genes. While CPs altered microbiome composition, most notably in healthy mice before colitis, these shifts did not correspond to changes in inflammatory signaling. Basal diet remained the primary determinant of inflammation, mucosal damage, and colitis severity in this model.