Soluble amyloid β-peptide oligomers (AβOs), increasingly recognized as causative agents of Alzheimer’s disease (AD), disrupt neuronal Ca²⁺ homeostasis and synaptic function. Here, we report that AβOs at sublethal concentrations generate prolonged Ca²⁺ signals in primary hippocampal neurons; incubation in Ca²⁺-free solutions, inhibition of ryanodine receptors (RyRs) or N-methyl-d-aspartate receptors (NMDARs), or preincubation with N-acetyl-l-cysteine abolished these signals. AβOs decreased (6 h) RyR2 and RyR3 mRNA and RyR2 protein, and promoted mitochondrial fragmentation after 24 h. NMDAR inhibition abolished the RyR2 decrease, whereas RyR inhibition prevented significantly the RyR2 protein decrease and mitochondrial fragmentation induced by AβOs. Incubation with AβOs (6 h) eliminated the RyR2 increase induced by brain-derived nerve factor (BDNF) and the dendritic spine remodeling induced within minutes by BDNF or the RyR agonist caffeine. Addition of BDNF to neurons incubated with AβOs for 24 h, which had RyR2 similar to and slightly higher RyR3 protein content than those of controls, induced dendritic spine growth but at slower rates than in controls. These combined effects of sublethal AβOs concentrations (which include redox-sensitive stimulation of RyR-mediated Ca²⁺ release, decreased RyR2 protein expression, mitochondrial fragmentation, and prevention of RyR-mediated spine remodeling) may contribute to impairing the synaptic plasticity in AD. Antioxid. Redox Signal. 14, 1209–1223.