Considerable fluctuations in cognitive performance and eventual dementia are an important characteristic of alpha-synucleinopathies, such as Parkinson’s disease (PD) and Lewy Body dementia (LBD) and are linked to cortical dysfunction. The presence of misfolded and aggregated alpha-synuclein (a-syn) in the cerebral cortex of patients has been suggested to play a crucial role in this process. However, the consequences of a-syn accumulation on the function of cortical networks at cellular resolution in vivo are largely unknown. Here we used the striatal seeding model in wildtype mice in order to induce robust a-synuclein pathology in the cerebral cortex. 9 months after a single intrastriatal injection of a-syn preformed fibrils, we performed in vivo two-photon calcium imaging in awake mice. We observed profound alterations of the function of layer 2/3 cortical neurons in somatosensory cortex (S1), as witnessed by an enhanced response to whisking and increased synchrony, accompanied by a decrease in baseline Ca2+ levels. Stereological analyses revealed a reduction in GAD67-positive inhibitory cells in S1 in PFF-injected brains. These findings point to a disturbed excitation/inhibition balance as an important driver of circuit dysfunction in alpha-synucleinopathies, which may underly cognitive changes in these diseases.