RAB5 GTPases act as molecular switches that regulate various endosomal functions in animal cells, including homotypic fusion of early endosomes, endosomal motility, endosomal signaling, and subcompartmentalization of the endosomal membrane. RAB5 proteins fulfill these diverse functions through interactions with downstream effector molecules. Two canonical RAB5 members, ARA7 and RAB HOMOLOG1 (RHA1), are encoded in the Arabidopsis thaliana genome. ARA7 and RHA1 play crucial roles in endocytic and vacuolar trafficking pathways. Plant RAB5 GTPases function via interactions with effector molecules, whose identities and functions are currently unclear. In this study, we searched for canonical RAB5 effector molecules of Arabidopsis and identified a candidate, which we called ENDOSOMAL RAB EFFECTOR WITH PX-DOMAIN (EREX). The intimate genetic interaction between EREX and RAB5 members, the results from subcellular colocalization experiments, and the direct interaction observed in an in vitro pull-down assay strongly suggest that EREX is a genuine effector of canonical RAB5s in Arabidopsis. We further found that close homologs of EREX play partially redundant functions with EREX in the transport of seed storage proteins. Our results indicate that canonical plant RAB5s acquired distinct effector molecules from those of non-plant systems to fulfill their functions.