J domain-containing proteins (J proteins) are functional partners for heat shock protein 70 (Hsp70) molecular chaperones and mediate various cellular processes by regulating activities of Hsp70. Budding yeast has three J proteins in the endoplasmic reticulum (ER): Scj1p and Jem1p functioning in protein folding and quality control in the ER, and Sec63p functioning in protein translocation across the ER membrane as partners for BiP, an Hsp70 in the ER. Here we report that Arabidopsis thaliana has orthologs of these yeast ER J proteins, which we designated as AtERdj3A, AtERdj3B, AtP58(IPK), AtERdj2A and AtERdj2B. Tunicamycin treatment of Arabidopsis cells, which causes ER stress, led to up-regulation of AtERdj3A, AtERdj3B, AtP58(IPK) and AtERdj2B. Subcellular fractionation analyses showed their ER localization, indicating that the identified J proteins indeed function as partners for BiP in Arabidopsis cells. Since expression of AtERdj3A, AtERdj3B and AtP58(IPK) partially suppressed the growth defects of the yeast jem1Deltascj1Delta mutant, they have functions similar to those of Scj1p and Jem1p. T-DNA insertions of the AtERDJ2A gene resulted in pollen germination defects, probably reflecting its essential function in protein translocation. These results suggest that A. thaliana has a set of ER J proteins structurally and functionally conserved from yeast to plants.