Transcriptional repressor, hypermethylated in cancer 1 (HIC1) is an important contributor to regulatory T (Treg) cell development and function. To investigate the mechanism by which it regulates Treg cell development, we systematically characterized the HIC1 interactome by affinity-purification mass spectrometry in human regulatory T cells. Interactors, involved in protein transport, mRNA processing, non-coding (ncRNA) transcription and RNA metabolism processes were identified. These data demonstrate that HIC1 is a part of a FOXP3-RUNX1-CBFβ protein complex that regulates Treg signature genes and is indispensable for the suppressive function of FOXP3+ regulatory T cells.