Malectin is a newly discovered endoplasmic reticulum (ER)-resident lectin, which specifically recognizes Glc2Man9GlcNAc2 on newly synthesized glycoproteins. We have previously demonstrated that malectin forms a complex with ribophorin I for selective retention of misfolded glycoproteins inside the cell. Here, we showed that ribophorin I also functions to regulate the subcellular localization of malectin under various conditions. Even though malectin does not contain an ER-retention signal motif, we found that endogenous malectin mainly localizes in the ER, which is disrupted upon suppression of ribophorin I, leading to its movement from ER to Golgi. In contrast, under ER-stress conditions, malectin mainly localizes in the Golgi, which is restored to ER localization by over-expression of ribophorin I. These results indicate that the subcellular localization of malectin is accurately regulated by the expression level of ribophorin I, which will provide further insights into the understanding of the function of malectin.