We isolated the cDNA of a novel protein disulfide isomerase (PDI)‐related protein, designated PDIR, from a human placental cDNA library. Deduced from its nucleotide sequence, PDIR has the three CXXC‐like motifs (Cys‐Ser‐Met‐Cys, Cys‐Gly‐His‐Cys and Cys‐Pro‐His‐Cys), which are found in proteins within the PDI superfamily and are responsible for oxidoreductase activity. PDIR has a hydrophobic stretch at its amino terminus, which may serve as a signal sequence, and the putative endoplasmic reticulum (ER) retention signal ‘Lys‐Glu‐Glu‐Leu’ at its carboxy terminus, indicating that PDIR is an ER resident protein. Northern blots showed that PDIR is preferentially expressed in cells actively secreting proteins and that the expression of PDIR is stress‐inducible. These results suggested that PDIR has oxidoreductase activity of disulfide bonds against polypeptides and that it acts as a catalyst of protein folding in the lumen of the ER.