The structure of the tryptophan synthase β2 subunit (Pfβ2) from the hyperthermophile, Pyrococcus furiosus, was determined by X-ray crystallographic analysis at 2.2 Å resolution, and its stability was examined by DSC. This is the first report of the X-ray structure of the tryptophan synthase β2 subunit alone, although the structure of the tryptophan synthase α2β2 complex from Salmonella typhimurium has already been reported. The structure of Pfβ2 was essentially similar to that of the β2 subunit (Stβ2) in the α2β2 complex from S. typhimurium. The sequence alignment with secondary structures of Pfβ and Stβ in monomeric form showed that six residues in the N-terminal region and three residues in the C-terminal region were deleted in Pfβ, and one residue at Pro366 of Stβ and at Ile63 of Pfβ was inserted. The denaturation temperature of Pfβ2 was higher by 35 °C than the reported values from mesophiles at ≈ pH 8. On the basis of structural information on both proteins, the analyses of the contributions of each stabilization factor indicate that: (a) the higher stability of Pfβ2 is not caused by either a hydrophobic interaction or an increase in ion pairs; (b) the number of hydrogen bonds involved in the main chains of Pfβ is greater by about 10% than that of Stβ, indicating that the secondary structures of Pfβ are more stabilized than those of Stβ and (c) the sequence of Pfβ seems to be better fitted to an ideally stable structure than that of Stβ, as assessed from X-ray structure data.