Oligomers of the β-amyloid (Aβ) peptide have been indicated in early neuropathologic changes in Alzheimer's disease. Here, we present a synthetic Aβ20-42 oligomer (named globulomer) with a different conformation to monomeric and fibrillar Aβ peptide, enabling the generation of highly Aβ oligomer-specific monoclonal antibodies. The globulomer-derived antibodies specifically detect oligomeric but not monomeric or fibrillar Aβ in various Aβ preparations. The globulomer-specific antibody A-887755 was able to prevent Aβ oligomer binding and dynamin cleavage in primary hippocampal neurons and to reverse globulomer-induced reduced synaptic transmission. In amyloid precursor protein (APP) transgenic mice, vaccination with Aβ globulomer and treatment with A-887755 improved novel object recognition. The cognitive improvement is likely attributable to reversing a deficit in hippocampal synaptic spine density in APP transgenic mice as observed after treatment with A-887755. Our findings demonstrate that selective reduction of Aβ oligomers by immunotherapy is sufficient to normalize cognitive behavior and synaptic deficits in APP transgenic mice.