The generation of regulatory T (Treg) cells is driven by Foxp3 and is responsible for dampening inflammation and reducing autoimmunity. In this study, the epigenetic regulation of inducible Treg (iTreg) cells was examined and an H3K4 histone methyltransferase, SMYD3 (SET and MYND Domain 3), which regulates the expression of Foxp3 by a TGFβ1/Smad3 (transforming growth factor-β1/Smad3)-dependent mechanism, was identified. Using chromatin immunoprecipitation assays, SMYD3 depletion led to a reduction in H3K4me3 in the promoter region and CNS1 (conserved noncoding DNA sequence) of the foxp3 locus. SMYD3 abrogation affected iTreg cell formation while allowing dysregulated interleukin-17 production. In a mouse model of respiratory syncytial virus (RSV) infection, a model in which iTreg cells have a critical role in regulating lung pathogenesis, SMYD3(-/-) mice demonstrated exacerbation of RSV-induced disease related to enhanced proinflammatory responses and worsened pathogenesis within the lung. Our data highlight a novel activation role for the TGFβ-inducible SMYD3 in regulating iTreg cell formation leading to increased severity of virus-related disease.