Abnormal folding and accumulation of α-synuclein is implicated in several neurological disorders including Parkinson's disease. Although α-synuclein is a typical cytoplasmic protein, a small amount of both monomeric and aggregated forms is secreted from cells and is present in human body fluids, such as cerebrospinal fluid. Extracellular α-synuclein aggregates have been shown to be neurotoxic, posing a challenge to any cell exposed to them. Here, we examine the internalization of various forms of extracellular α-synuclein, including fibrils, oligomers, and monomer, into neuronal cells and their subsequent degradation. Internalization of fibrillar α-synuclein could be inhibited by low temperature or the expression of a dominant-negative mutant dynamin-1 K44A, suggesting the endocytosis-mediated internalization. The internalized fibrils moved through the endosomal pathway and were degraded in the lysosome, which ultimately resulted in the clearance of the α-synuclein aggregates from the culture medium. Non-fibrillar oligomeric aggregates were also internalized via endocytosis and degraded by the lysosome. In contrast to aggregate uptake, the internalization of monomeric α-synuclein was unaffected by cold temperature and the expression of dynamin-1 K44A, consistent with direct translocation across the plasma membrane. Internalized monomers rapidly pass the plasma membrane, escaping the cells before being degraded by the cellular proteolytic systems. These results suggest that only aggregated forms of extracellular α-synuclein can be cleared by cell-mediated uptake and degradation, and this might represent a mechanism of preventing neurons from exposure to potentially toxic α-synuclein.