Pathogenic variants in PRKN cause early‐onset Parkinson's disease (PD), while the role of alpha‐synuclein in PRKN‐PD remains uncertain. One study performed a blood‐based alpha‐synuclein seed amplification assay (SAA) in PRKN‐PD, not detecting seed amplification in 17 PRKN‐PD patients. By applying a methodologically different SAA focusing on neuron‐derived extracellular vesicles, we demonstrated alpha‐synuclein seed amplification in 8 of 13 PRKN‐PD patients, challenging the view of PRKN‐PD as a non‐synucleinopathy. Moreover, we performed blinded replication of the neuron‐derived extracellular vesicles‐dependent SAA in idiopathic PD patients and healthy controls. In conclusion, blood‐based neuron‐derived extracellular vesicles‐dependent SAA represents a promising biomarker to elucidate the underpinnings of (monogenic) PD. ANN NEUROL 2024;95:1173–1177