Ribonucleotide reductases (RNRs) catalyze the reduction of ribonucleotides to the corresponding deoxyribonucleotides. The catalytic activity of most RNRs depends on the formation of a dimer of the catalytic subunits. The active site is located at the interface, and part of the substrate binding site and regulatory mechanisms work across the subunit in the dimer. In this study, we describe and characterize a novel domain responsible for forming the catalytic dimer in several class II RNRs. The 3D structure of the class II RNR from Rhodobacter sphaeroides reveals a so far undescribed α-helical domain in the dimer interface, which is embracing the other subunit. Genetic removal of this HUG domain leads to a severe reduction of activity paired with reduced dimerization capability. In comparison with other described RNRs, the enzyme with this domain is less dependent on the presence of nucleotides to act as allosteric effectors in the formation of dimers. The HUG domain appears to serve as an interlock to keep the dimer intact and functional even at low enzyme and/or effector concentrations.