Small heat shock proteins (sHsps) are the most ubiquitous molecular chaperones. Several sHsps have been shown to exhibit chaperone activity and protect proteins from thermal and chemical aggregation. We have characterized a small heat shock protein from a hyperthermophilic archaeum, Thermococcus sp. strain KS-1. Electron microscopy revealed that the protein exists as a spherical oligomer with a diameter of 14 +/-1 nm. The molecular weight of the oligomer was determined to be 478.6 kDa by size exclusion chromatography-multiangle laser light scattering. Thus, the Thermococeus sHsp is likely to exist as a spherical 24meric oligomer with almost the same structure as the Methanococcus jannaschii sHsp. The Thermococcus sHsp homo-oligomer protected porcine heart citrate synthase from thermal aggregation. It also slightly enhanced the refolding of acid-denatured green fluorescent protein. While the Thermococcus sHsp could not be detected in cells grown at the optimal growth temperature or lower, the expression of the protein was highly induced when the cells were grown at temperatures higher than the optimal growth temperature. Since only group H chaperonins and sHsps exist in hyperthermophilic archaea as molecular chaperones, sHsps should have an important role in protecting cells from lesions caused by aggregates of thermally denatured cellular proteins.