Regulation of transcription requires cooperation between sequence-specific transcription factors and numerous coregulatory proteins. In IL-4/IL-13 signaling several coactivators for STAT6 have been identified, but the molecular mechanisms of STAT6-mediated gene transcription are still not fully understood. Here we identified by proteomic approach that the PTB-associated splicing factor (PSF) interacts with STAT6. In intact cells the interaction was observed only after IL-4 stimulation. The IL-4-induced tyrosine phosphorylation of both STAT6 and PSF is a prerequisite for the efficient association of the two proteins. Functional analysis demonstrated that ectopic expression of PSF resulted in inhibition of STAT6-mediated transcriptional activation and mRNA expression of the Igε germline heavy chain gene, whereas knockdown of PSF increased the STAT6-mediated responses. PSF recruited histone deacetylase 1 (HDAC1) to the STAT6 transcription complex, which resulted in reduction of H3 acetylation at the promoter regions of Ig heavy chain germline Igε and inhibition of STAT6-mediated transcription. In addition, the HDACs inhibitor trichostatin A (TSA) enhanced H3 acetylation, and reverted the PSF-mediated transcriptional repression of Igε gene transcription. In summary, these results identify PSF as a repressor of STAT6-mediated transcription that functions through recruitment of HDAC to the STAT6 transcription complex, and delineates a novel regulatory mechanism of IL-4 signaling that may have implications in the pathogenesis of allergic diseases and pharmacological HDAC inhibition in lymphomas.