Amyloid β-protein (Aβ) pathologies have been linked to dysfunction of excitability in neurons of the hippocampal circuit, but the molecular mechanisms underlying this process are still poorly understood. Here we applied whole-cell patch-clamp electrophysiology to primary hippocampal neurons and show that intracellular Aβ42 delivery leads to increased spike discharge and action potential broadening through down-regulation of A-type K+ currents. Pharmacological studies showed that caspases and GSK-3 activation are required for these Aβ42-induced effects. Extracellular perfusion and subsequent internalization of Aβ42 increase spike discharge and promote GSK-3-dependent phosphorylation of the Kv4.2 α-subunit, a molecular determinant of A-type K+ currents, at Ser-616. In acute hippocampal slices derived from an adult triple-transgenic Alzheimer’s mouse model (3xTg-AD), characterized by endogenous intracellular accumulation of Aβ42, CA1 pyramidal neurons exhibit hyperexcitability accompanied by increased phosphorylation of Kv4.2 at Ser-616. Collectively, these data suggest that intraneuronal Aβ42 accumulation leads to an intracellular cascade culminating into caspases activation and GSK-3-dependent phosphorylation of Kv4.2 channels. These findings provide new insights into the toxic mechanisms triggered by intracellular Aβ42 and offer potentially new therapeutic targets for Alzheimer’s disease (AD) treatment.