Purpose: The etiology and pathogenesis of distal colitis (DC) are poorly understood. Activation of intestinal inflammatory response may lead to intestinal tissue necrosis. Antioxidant and anti-inflammatory agents are among the treatment options. Our study aimed to compare the protective effects of mesalazine and Ganoderma lucidum in acetic acid (AA)-induced colitis in rats. Methods: Twenty-four rats were randomly grouped as colitis, mesalazine, G. lucidum, and combined (G. lucidum + mesalazine) groups. DC was induced by intrarectal administration of AA. Statistical comparisons were done by using parameters including colonic tissue IL-1, IL-6, TNF-α, and CRP levels. Histopathologic changes of the samples of colonic tissue were scored as mucosal damage score and inflammatory score. A P-value of <0.05 was considered significant. Results: Intrarectal administration of AA leads to increased interleukin and CRP levels. High mucosal damage and inflammatory scores were noted in colitis group animals. Single mesalazine or G. lucidum treatment produced considerably decreased tissue interleukin and CRP levels. The lowest tissue interleukin and CRP levels were noted in the combined treatment group of animals. Mucosal damage and inflammatory scores were found to be significantly low in this group of animals. Conclusion The intrarectal administration of AA results in an activation of intestinal inflammation and severe mucosal damage in colonic tissue. Single use of mesalazine and G. lucidum treatment decreases the severity of intestinal inflammatory response and mucosal damage. The healing effects of the combined treatment of mesalazine and G. lucidum seem to be more effective than that of separate use in the treatment of DC.