There is great interest in safe and effective alternative therapies that could benefit patients with inflammatory bowel diseases (IBD). L-arginine (Arg) is a semi-essential amino acid with a variety of physiological effects. In this context, our aim was to investigate the role of dietary Arg in experimental colitis. We used two models of colitis in C57BL/6 mice, the dextran sulfate sodium (DSS) model of injury and repair, and infection. Animals were given diets containing (1) no Arg (Arg), 6.4 g/kg (Arg), or 24.6 g/kg Arg (Arg); or (2) the amino acids downstream of Arg: 28 g/kg L-ornithine (Orn) or 72 g/kg L-proline (Pro). Mice with DSS colitis receiving the Arg diet had increased levels of Arg, Orn, and Pro in the colon and improved body weight loss, colon length shortening, and histological injury compared to Arg and Arg diets. Histology was improved in the Arg vs. Arg group. Orn or Pro diets did not provide protection. Reduction in colitis with Arg diet also occurred in -infected mice. Diversity of the intestinal microbiota was significantly enhanced in mice on the Arg diet compared to the Arg or Arg diets, with increased abundance of Bacteroidetes and decreased Verrucomicrobia. In conclusion, dietary supplementation of Arg is protective in colitis models. This may occur by restoring overall microbial diversity and Bacteroidetes prevalence. Our data provide a rationale for Arg as an adjunctive therapy in IBD.