Cytoplasmic dynein drives the movement of a wide range of cargoes towards the minus ends of microtubules. We previously demonstrated that LIS1 forms an idling complex with dynein, which is transported to the plus ends of microtubules by kinesin motors. Here we report that the small GTPase Rab6a is essential for activation of idling dynein. Immunoprecipitation and microtubule pull-down assays reveal that the GTP bound mutant, Rab6a(Q72L), dissociates LIS1 from a LIS1-dynein complex, activating dynein movement in in vitro microtubule gliding assays. We monitor transient interaction between Rab6a(Q72L) and dynein in vivo using dual-colour fluorescence cross-correlation spectroscopy in dorsal root ganglion (DRG) neurons. Finally, we demonstrate that Rab6a(Q72L) mediates LIS1 release from a LIS1-dynein complex followed by dynein activation through an in vitro single-molecule assay using triple-colour quantum dots. Our findings reveal a surprising function for GTP bound Rab6a as an activator of idling dynein.