The ligand activated transcription factor aryl hydrocarbon receptor (AhR) has been studied for many decades in toxicology as the ligand for the environmental contaminant dioxin. However, AhR has recently emerged as a critical physiological regulator of immune responses affecting both innate and adaptive systems, and several AhR ligands with different pharmacological profiles have recently been studied. The current review discusses new insights into the role of AhR signalling and AhR ligands on the regulation of the immune system, with a focus on regulatory T cells which maintain immune tolerance. Notably, AhR is expressed and modulates the development of two induced regulatory CD4+ T cell subsets, the forkhead box P3-positive (Foxp3+) regulatory T cells (iTreg) and the IL-10-secreting type 1 regulatory T (T(R)1) cells, through different signalling pathways. We will finally discuss how AhR ligands could be exploited to alleviate human autoimmune diseases. Clearly, drugs targeted against AhR should promote the development of new strategies to fight against autoimmune diseases.