P5, also known as PDIA6, is a PDI family member involved in the ER quality control. Here, we revealed that P5 dimerizes via a unique adhesive motif contained in the N-terminal thioredoxin-like domain. Unlike conventional leucine zipper motifs with leucine residues every two helical turns on ∼30-residue parallel α helices, this adhesive motif includes periodic repeats of leucine/valine residues at the third or fourth position spanning five helical turns on 15-residue anti-parallel α helices. The P5 dimerization interface is further stabilized by several reciprocal salt bridges and C-capping interactions between protomers. A monomeric P5 mutant with the impaired adhesive motif showed structural instability and local unfolding, and behaved as aberrant proteins that induce the ER stress response. Disassembly of P5 to monomers compromised its ability to inactivate IRE1α via intermolecular disulfide bond reduction and its Ca2+-dependent regulation of chaperone function in vitro. Thus, the leucine-valine adhesive motif supports structure and function of P5. Display omitted •P5 dimerizes via a unique leucine-valine adhesive motif in the first Trx-like domain•The adhesive motif is radically different from conventional coiled-coil motifs•The dimeric structure serves to promote P5-mediated inactivation of IRE1α•Ca2+-binding regions and Ca2+-dependent functional regulation were clarified for P5 Using SAXS and X-ray crystallography, Okumura et al. determined the structure of a protein disulfide isomerase P5, in which the N-terminal thioredoxin-like domain dimerizes via a unique leucine-valine adhesive motif. A monomeric P5 mutant displayed structural instability and compromised functions, indicating the structural and functional essentiality of this motif.