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Regulatory B cells (Bregs) play a critical role in the control of autoimmunity and inflammation. IL-10 production is the hallmark for the identification of Bregs. However, the molecular determinants that regulate the transcription of IL-10 and control the Breg developmental program remain unknown. The aryl hydrocarbon receptor (AHR) is an environmental sensor that binds to a variety of ligands, including physiological compounds derived from the digestion of dietary components by commensal microbiota. Here, we demonstrate that AHR regulates the differentiation and function of IL-10-producing CD19+CD21hiCD24hi Bregs and limits their differentiation into B cells that contribute to inflammation. Chromatin profiling and transcriptome analyses show that loss of AHR in B cells reduces expression of IL-10 by skewing the differentiation of CD19+CD21hiCD24hi B cells into a pro-inflammatory program, under Breg-inducing conditions. B cell AHR-deficient mice develop exacerbated arthritis, show significant reductions in IL-10-producing Bregs and regulatory T cells (Tregs), and show an increase in T helper (Th) 1 and Th17 cells compared with B cell AHR-sufficient mice. The most abundant source of AHR ligands are derived from the diet and the metabolism of dietary tryptophan. We have previously established a link between microbiota-driven signals in the gut and the differentiation of Bregs. Of the gut microbiota-derived metabolites, the short-chain fatty acids (SCFAs) are the most well characterised. More recently, the SCFA butyrate has been demonstrated to act as an AHR ligand in an intestinal epithelial cell line. Given the association of butyrate with AHR activation and the supporting findings showing that butyrate promotes Treg function, these data led us to hypothesise that butyrate acts via AHR to enhance Breg suppression. Here, we demonstrate that mice supplemented with butyrate reduces arthritis severity by inhibiting the differentiation of GC B cells and plasma cells, whilst maintaining Breg numbers and promoting the suppressive function of Bregs. We show that supplementation of mice with butyrate, changes the composition of the microbiota to favour species which metabolise tryptophan; a major source of AHR ligands. Therefore, we hypothesised that butyrate controls the balance between pro-arthritogenic and regulatory B cell differentiation, through the generation of microbiota derived AHR ligands. To date, the AHR ligands which direct Breg function are unknown. We rule out that butyrate acts as a direct ligand of AHR and establish that supplementation with butyrate increases the availability of 5-Hydroxyindole-3-acetic acid (5-HIAA), a downstream metabolite of serotonin, which we identify as a novel ligand of AHR in B cells. Mice supplemented with 5-HIAA promote Breg function and suppress arthritis severity, only in mice with AHR-sufficient B cells. Thus, we identify AHR as a relevant contributor to the transcriptional regulation of Breg differentiation and show that microbiota in the gut influence Breg differentiation by increasing the availability of AHR ligands.