The Sin3A complex acts as a transcriptional hub, integrating the function of diverse transcription factors with histone modifying enzymes, notably, histone deacetylases (HDAC) 1 and 2. The Sin3A protein sits at the centre of the complex, mediating multiple simultaneous protein-protein interactions via its four paired-amphipathic helix (PAH) domains (PAH1-4). The PAH domains contain a conserved four helical bundle, generating a hydrophobic cleft into which the single-helix of a Sin3-interaction domain (SID) is able to insert and bind with high affinity. Although they share a similar mode of interaction, the SIDs of different repressor proteins bind to only one of four potential PAH domains, due to the specific combination of hydrophobic residues at the interface. Here we report the identification of a highly conserved SID in the 5-methylcytosine dioxygenase, Tet1 (Tet1-SID), which interacts directly with the PAH1 domain of Sin3A. Using a combination of NMR spectroscopy and homology modelling we present a model of the PAH1/Tet1-SID complex, which binds in a Type-II orientation similar to Sap25. Mutagenesis of key residues show that the 11-amino acid Tet1-SID is necessary and sufficient for the interaction with Sin3A and is absolutely required for Tet1 to repress transcription in cells.