Parkinson disease (PD)-associated mutations in the LRRK2 gene hyperactivate LRRK2 kinase activity, leading to increased phosphorylation of a subset of RAB GTPases, which are master regulators of intracellular trafficking. In neurons, processive retrograde transport of autophagosomes is essential for autophagosome maturation and effective degradation of autophagosomal cargo in the axon. Here, we show that knockout of the LRRK2-counteracting RAB phosphatase PPM1H causes a gene-dose-dependent disruption of the axonal transport of autophagosomes, leading to impaired degradation of axonal alpha-synuclein (aSyn), a key protein in PD pathophysiology. Defective autophagosome transport and impaired aSyn degradation correlate with increased aSyn aggregation in primary PPM1H knockout neurons exposed to preformed fibrils of aSyn, an effect that is dependent on LRRK2 kinase activity. These findings mechanistically link LRRK2-mediated RAB hyperphosphorylation to defective autophagosomal degradation and enhanced aggregation of aSyn, positioning the LRRK2-RAB axis as a key driver of PD pathophysiology.