AbstractEnvironmental factors are the major contributor to the onset of immunological disorders such as ulcerative colitis. However, their identities remain unclear. Here, we discover that the amount of consumed L-Tryptophan (L-Trp), a ubiquitous dietary component, determines the transcription level of the colonic T cell homing receptor, GPR15, hence affecting the number of colonic FOXP3⁺ regulatory T (Treg) cells and local immune homeostasis. Ingested L-Trp is converted by host IDO1/2 enzymes, but not by gut microbiota, to compounds that induce GPR15 transcription preferentially in Treg cells via the aryl hydrocarbon receptor. Consequently, two weeks of dietary L-Trp supplementation nearly double the colonic GPR15⁺ Treg cells via GPR15-mediated homing and substantially reduce the future risk of colitis. In addition, humans consume 3–4 times less L-Trp per kilogram of body weight and have fewer colonic GPR15⁺ Treg cells than mice. Thus, we uncover a microbiota-independent mechanism linking dietary L-Trp and colonic Treg cells, that may have therapeutic potential.