The most prevalent form of inflammatory bowel disease (IBD), ulcerative colitis (UC), is characterized by persistent inflammation of the colorectal mucosa. It is asymptomatic, whereas Crohn's disease (CD) causes patchy lesions in the gastrointestinal tract. Men and women suffer equally from ulcerative colitis, which usually strikes in the second and third decades of life and becomes more common in senior citizens. In the present study, we produced zinc oxide nanoparticles using the natural herbal plant, Cassia alata. Zinc oxide nanoparticles have remarkable antimicrobial and antitumor benefits in the field of biomedical science. Furthermore, the synthesized zinc oxide nanoparticles (ZnO NPs) were characterized using UV, XRD, FTIR, and SEM analyses. The XRD analysis confirmed the crystallite nature and purity of the synthesized nanoparticles. Zinc oxide nanoparticles with a uniform size and partially agglomerated morphology were verified by SEM analysis. We investigated the protective effects of environmentally friendly zinc oxide nanoparticles in dextran sodium sulfate-induced ulcerative colitis mouse models. Green synthesized Cassia alata zinc oxide nanoparticles (CA ZnO NPs) reversed weight loss, disease activity index, colon shortening, and colon histological damage. Zinc oxide nanoparticles reduce hypersensitivity, oxidative stress, and inflammation, and protect the mucosal layer. Green synthesized CA ZnO NPs demonstrated protection against dextran sodium sulfate-induced ulcerative colitis via anti-inflammatory activity.