Dimeric RING E3 ligases interact with protein substrates and conformationally restrain the ubiquitin-E2-conjugating enzyme thioester complex such that it is primed for catalysis. RNF4 is an E3 ligase containing an N-terminal domain that binds its polySUMO substrates and a C-terminal RING domain responsible for dimerization. To investigate how RNF4 activity is controlled, we increased polySUMO substrate concentration by ablating expression of SUMO protease SENP6. Accumulation of SUMO chains in vivo leads to ubiquitin-mediated proteolysis of RNF4. In vitro we demonstrate that at concentrations equivalent to those found in vivo RNF4 is predominantly monomeric and inactive as an ubiquitin E3 ligase. However, in the presence of SUMO chains, RNF4 is activated by dimerization, leading to both substrate ubiquitylation and autoubiquitylation, responsible for degradation of RNF4. Thus the ubiquitin E3 ligase activity of RNF4 is directly linked to the availability of its polySUMO substrates. Display omitted •Characterization of the mechanism of activation of the ubiquitin E3 ligase RNF4•First indication of E3 ligase activation by SUMO chains RNF4 is an E3 ligase containing an N-terminal domain that binds its polySUMO substrates and a C-terminal RING domain responsible for dimerization. Rojas-Fernandez et al. show that in the presence of SUMO chains RNF4 is activated by dimerization, leading to both substrate ubiquitylation and autoubiquitylation.