Small GTPase Rab17 has recently been shown to regulate dendritic morphogenesis of mouse hippocampal neurons; however, the exact molecular mechanism of Rab17-mediated dendritogenesis remained to be determined, because no guanine nucleotide exchange factor (GEF) for Rab17 had been identified. In this study we screened for the Rab17-GEF by performing yeast two-hybrid assays with a GDP-locked Rab17 mutant as bait and found that Rabex-5 and ALS2, both of which were originally described as Rab5-GEFs, interact with Rab17. We also found that expression of Rabex-5, but not of ALS2, promotes translocation of Rab17 from the cell body to the dendrites of developing mouse hippocampal neurons. The shRNA-mediated knockdown of Rabex-5 or its known downstream target Rab5 in hippocampal neurons inhibited morphogenesis of both axons and dendrites, whereas knockdown of Rab17 affected dendrite morphogenesis alone. Based on these findings, we propose that Rabex-5 regulates neurite morphogenesis of hippocampal neurons by activating at least two downstream targets, Rab5, which is localized in both axons and dendrites, and Rab17, which is localized in dendrites alone.