Chitinase D (designated as Pc-ChiD) was found in a hyperthermophilic archaeon, Pyrococcus chitonophagus (previously described as Thermococcus chitonophagus), that was isolated from media containing only chitin as carbon source. Pc-ChiD displays chitinase activity and is thermostable at temperatures up to 95˚C, suggesting its potential for industrial use. Pc-ChiD has a secretion signal peptide and two chitin-binding domains (ChBDs) in the N-terminal domain. However, the C-terminal domain shares no sequence similarity with previously identified saccharide-degrading enzymes, and does not contain the DXDXE motif conserved in the glycoside hydrolase (GH) 18-family chitinases. To elucidate its overall structure and reaction mechanism, we determined the first crystal structures of Pc-ChiD, both in the ligand-free form and in complexes with substrates. Structure analyses revealed that the C-terminal domain of Pc-ChiD, Pc-ChiD(ΔBD), consists of a third putative substrate-binding domain, which cannot be predicted from the amino acid sequence, and a catalytic domain structurally similar to that found in not the GH18 family but the GH23 family. Based on the similarity with GH23-family chitinase, the catalytic residues of Pc-ChiD were predicted and confirmed by mutagenesis analyses. Moreover, the specific C-terminal 100 residues of Pc-ChiD are important to fix the putative substrate-binding domain next to the catalytic domain, contributing to the structure stability as well as the long chitin-chain binding. Our findings reveal the structure of a unique archaeal chitinase that is distinct from previously known members of the GH23 family.