Although misfolded and aggregated alpha-synuclein (alpha-syn) is recognized in the disease progression of synucleinopathies, its role in the impairment of cortical circuitries and synaptic plasticity remains incompletely understood. We investigated how alpha-synuclein accumulation affects synaptic plasticity in the mouse somatosensory cortex using two distinct approaches. Long-term in vivo imaging of apical dendrites was performed in mice overexpressing wild-type human alpha-synuclein. Additionally, intracranial injection of preformed alpha-synuclein fibrils was performed to induce cortical alpha-syn pathology. We find that alpha-synuclein overexpressing mice show decreased spine density and abnormalities in spine dynamics in an age-dependent manner. We also provide evidence for the detrimental effects of seeded alpha-synuclein aggregates on dendritic architecture. We observed spine loss as well as dystrophic deformation of dendritic shafts in layer V pyramidal neurons. Our results provide a link to the pathophysiology underlying dementia associated with synucleinopathies and may enable the evaluation of potential drug candidates on dendritic spine pathology in vivo.