TET proteins oxidize 5-methylcytosine (mC) to 5-hydroxymethylcytosine (hmC), 5-formylcytosine (fC) and 5-carboxylcytosine (caC) and thus provide a possible means for active DNA demethylation in mammals. Although their catalytic mechanism is well characterized and the catalytic dioxygenase domain is highly conserved, the function of the regulatory regions - the N-terminus and the low complexity insert between the two parts of the dioxygenase domains - is only poorly understood. Here, we demonstrate that TET proteins are subject to a variety of PTMs that mostly occur at these regulatory regions. We mapped TET modification sites at amino acid resolution and show for the first time that TET1, TET2, and TET3 are highly phosphorylated. The glycosyltransferase OGT, which we identified as a strong interactor of all three TET proteins, catalyzes the addition of an N-acetylglucosamine (GlcNAc) group to serine and threonine residues of TET proteins and thereby decreases both the number of phosphorylation sites as well as the site occupancy. Interestingly, the different TET proteins display unique PTM patterns and some modifications occur in distinct combinations. In summary, our results provide a novel potential mechanism for TET protein regulation based on a dynamic interplay of phosphorylation and O-GlcNAcylation at the N-terminus and the low complexity insert region. Our data suggest strong crosstalk between the modification sites that could allow rapid adaption of TET protein localization, activity, or targeting due to changing environmental conditions as well as in response to external stimuli.