Synucleinopathies are neurodegenerative diseases characterized by the presence of brain inclusions containing the pathologically aggregated protein α-synuclein. The development of a positron emission tomography tracer to detect aggregates of misfolded α-synuclein could revolutionize early diagnosis, disease monitoring, and the evaluation of therapeutic efficacy. Here, we present the development, preclinical validation, and first-in-human evaluation of [11C]MODAG-005. In vitro binding experiments demonstrated subnanomolar binding affinity to recombinant α-synuclein fibrils and to α-synuclein inclusions in human brain tissue. Specific binding in multiple system atrophy (MSA) brain tissue was detected using autoradiography and microautoradiography and was validated through immunostaining. In vivo, [11C]MODAG-005 showed good brain penetration, rapid clearance from brain tissue, and low metabolite formation in rodents and nonhuman primates. In addition, a pronounced binding and a good signal-to-noise ratio were achieved in an α-synuclein fibril-injected rat model and in an α-synuclein (A30P) transgenic mouse model in correlation to the pathological load. To validate the potential of [11C]MODAG-005 for therapeutic development, we showed target engagement of the drug candidate anle138b in the brain tissues from α-synuclein (A30P) mice and patients with multiple system atrophy as well as in vivo in α-synuclein fibril-injected rats. Last, first-in-human imaging demonstrated [11C]MODAG-005 binding in brain regions affected by α-synuclein pathology in patients with clinically established MSA cerebellar type, MSA cerebellar and parkinsonian type, and Parkinson's disease.