Although the development of T-cell subsets is mainly regulated by a master transcriptional regulator and phosphorylation of the STAT protein in response to distinct cytokine stimulation, accumulating data indicate that other signaling pathways are also involved in regulating or fine-tuning T-cell lineage commitment. In this report, we investigated the role of ERK, mitogen-activated protein kinase (MAPK), in Th17 and Treg cell development. We demonstrate that blockade of ERK activation inhibited Th17-cell development while upregulating Treg cells under Th17 polarization conditions. Inhibition of ERK decreased IL-6 induction of RAR-related orphan receptor γt but enhanced TGF-β induction of Foxp3, and ERK inhibitor-treated T cells under Th17 conditions possessed suppressive function in vitro because they produced more IL-10 and TGF-β and inhibited naïve T-cell proliferation and IFN-γ production at levels comparable with that of Treg cells. Furthermore, ERK inhibitor-treated T cells under Th17 polarization conditions had a decreased potency to induce colitis in vivo. Collectively, our data demonstrated that the ERK pathway differentially regulates Th17- and Treg-cell differentiation, and thus interfering with the ERK pathway could represent a therapeutic treatment for inflammatory bowel diseases and other Th17-related autoimmune diseases. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.