The genetic basis of stress resistance in extremophilic microalgae is not well studied. In this study， a gene of unknown function， the cluster58 or CL58 gene， was identified from the halotolerant green alga Chlamydomonas W80 and characterized. The CL58 gene encodes a protein containing a domain of unknown function， the CHRD domain， and a putative secretory signaling sequence at its N-terminus. The levels of CL58 mRNA increased in response to high copper levels and low temperatures. When the CL58 gene was heterologously expressed as a fusion gene with the NanoLuc luciferase gene in Chlamydomonas reinhardtii， a majority of the NanoLuc activity was detected in the culture medium compared with that in the intracellular fraction. A mutagenic analysis revealed that the putative secretory signaling sequence was sufficient for the secretion of the CL58-NanoLuc fusion protein. In addition， we expressed the protein encoded by the CL58 gene in Escherichia coli; the recombinant， soluble protein was then purified. In summary， we identified a novel gene from C. W80 that appears to encode a stress-responsive， CHRD domain-containing secreted protein.