We isolated a protein, P45, from the extreme halophilic archaeon Haloarcula marismortui, which displays molecular chaperone activities in vitro. P45 is a weak ATPase that assembles into a large ring-shaped oligomeric complex comprising about 10 subunits. The protein shows no significant homology to any known protein. P45 forms complexes with halophilic malate dehydrogenase during its salt-dependent denaturation/renaturation and decreases the rate of deactivation of the enzyme in an ATP-dependent manner. Compared with other halophilic proteins, the P45 complex appears to be much less dependent on salt for its various activities or stability. In vivo experiments showed that P45 accumulates when cells are exposed to a low salt environment. We suggest, therefore, that P45 could protect halophilic proteins against denaturation under conditions of cellular hyposaline stress.