Amyloid-beta (Aβ) oligomers are thought to trigger Alzheimer’s disease (AD) pathophysiology. Cellular Prion Protein (PrP C ) selectively binds oligomeric Aβ and can mediate AD-related phenotypes. Here, we examined the specificity, distribution and signaling from Aβ/PrP complexes, seeking to explain how they might alter the function of NMDA receptors in neurons. PrP C is enriched in post-synaptic densities, and Aβ/PrP C interaction leads to Fyn kinase activation. Soluble Aβ assemblies derived from human AD brain interact with PrP C to activate Fyn. Aβ engagement of PrP C /Fyn signaling yields phosphorylation of the NR2B subunit of NMDA-receptors, which is coupled to an initial increase and then loss of surface NMDA-receptors. Aβ-induced LDH release and dendritic spine loss require both PrP C and Fyn, and human familial AD transgene-induced convulsive seizures do not occur in mice lacking PrP C . These results delineate an Aβ oligomer signal transduction pathway requiring PrP C and Fyn to alter synaptic function with relevance to AD.