Objective To investigate the role of macrophage-derived Act1 (nuclear factor kappa B activator 1) in the inflammatory bowel disease. Methods Genetically engineered mice carrying targeted suppression of Act1 in the macrophages (Anti-Act1) were used for the dextran sodium sulfate (DSS)-induced ulcerative colitis. The severity of colitis was assessed by weight loss, stool consistency, fecal blood index, colorectal length and H&E histology. The infiltration of CD45⁺ leukocytes and CD68⁺ macrophages in the inflammatory intestine was observed by immunohistochemical staining and expression levels of mRNA for inflammatory cytokines in colon tissues were analyzed by RT-qPCR. Results As compared with C57 mice, the anti-Act1 mice exhibited less severe acute colitis following DSS treatment, with reduced CD45⁺ leukocyte and CD68⁺ macrophage infiltrates in the colon tissue. Inflamed colons of the anti-Act1 mice expressed lower mRNA levels of TNF-α, IL-1β and IL-6. Conclusions Targeted suppression of Act1 in the macrophages ameliorates dextran sodium sulfate-induced intestinal inflammation.