Excessive accumulation of unfolded proteins in the endoplasmic reticulum (ER) lumen causes ER stress, which induces a set of genes, including those encoding ER-resident chaperones, to relieve the detrimental effects and recover homeostasis. Calreticulin is a chaperone that facilitates protein folding in the ER lumen, and its gene expression is induced by ER stress in Caenorhabditis elegans. Sumoylation conjugates small ubiquitin-like modifier (SUMO) proteins with target proteins to regulate a variety of biological processes, such as protein stability, nuclear transport, DNA binding, and gene expression. In this study, we showed that C. elegans X-box-binding protein 1 (Ce-XBP-1), an ER stress response transcription factor, interacts with the SUMO-conjugating enzyme UBC-9 and a SUMOylation target. Our results indicated that abolishing sumoylation enhanced calreticulin expression in an XBP-1-dependent manner, and the resulting increase in calreticulin counteracted ER stress. Furthermore, sumoylation was repressed in C. elegans undergoing ER stress. Finally, RNAi against ubc-9 mainly affected the expression of genes associated with ER functions, such as lipid and organic acid metabolism. Our results suggest that sumoylation plays a regulatory role in ER function by controlling the expression of genes required for ER homeostasis in C. elegans.