Intraneuronal tau aggregations are distinctive pathological features of Parkinson's disease (PD) with autosomal-dominant mutations inleucine-rich repeat kinase 2(LRRK2). The most prevalentLRRK2mutation, G2019S (glycine to serine substitution at amino acid 2019), causes neurite shrinkage through unclear pathogenetic mechanisms. We found that expression of G2019S mutant inDrosophiladendritic arborization neurons induces mislocalization of the axonal protein tau in dendrites and causes dendrite degeneration. G2019S-induced dendrite degeneration is suppressed by reducing the level of tau protein and aggravated by tau coexpression. Additional genetic analyses suggest that G2019S and tau function synergistically to cause microtubule fragmentation, inclusion formation, and dendrite degeneration. Mechanistically, hyperactivated G2019S promotes tau phosphorylation at the T212 site by theDrosophilaglycogen synthase kinase 3β homolog Shaggy (Sgg). G2019S increases the recruitment of autoactivated Sgg, thus inducing hyperphosphorylation and mislocalization of tau with resultant dendrite degeneration.