The chaperonins (CPNs) are megadalton sized hollow complexes with two cavities that open and close to encapsulate non-native proteins. CPNs are assigned to two sequence-related groups that have distinct allosteric mechanisms. In Group I CPNs a detachable co-chaperone, GroES, closes the chambers whereas in Group II a built-in lid closes the chambers. Group I CPNs have a bacterial ancestry, whereas Group II CPNs are archaeal in origin. Here we describe open and closed crystal structures representing a new phylogenetic branch of CPNs. These Group III CPNs are divergent in sequence and structure from extant CPNs, but are closed by a built-in lid like Group II CPNs. A nucleotide-sensing loop, present in both Group I and Group II CPNs, is notably absent. We identified inter-ring pivot joints that articulate during ring closure. These Group III CPNs likely represent a relic from the ancestral CPN that formed distinct bacterial and archaeal branches.Chaperonins (CPNs) are ATP-dependent protein-folding machines. Here the authors present the open and closed crystal structures of a Group III CPN from the thermophilic bacterium Carboxydothermus hydrogenoformans, discuss its mechanism and structurally compare it with Group I and II CPNs.