Age‐related macular degeneration (AMD) is characterized by the formation of drusen, extracellular deposits associated with atrophy of the retinal pigmented epithelium (RPE), disturbance of the transepithelial barrier and photoreceptor death. Amyloid‐β (Aβ) is present in drusen but its role during AMD remains unknown. This study investigated the in vitro and in vivo effects of the oligomeric form of Aβ(1‐42) – OAβ(1‐42) – on RPE and found that it reduced mitochondrial redox potential and increased the production of reactive oxygen species, but did not induce apoptosis in RPE cell cultures. It also disorganized the actin cytoskeleton and halved occludin expression, markedly decreasing attachment capacity and abolishing the selectivity of RPE cell transepithelial permeability. Antioxidant pretreatment partially reversed the effects of OAβ(1‐42) on mitochondrial redox potential and transepithelial permeability. Subretinally injected OAβ(1‐42) induced pigmentation loss and RPE hypertrophy but not RPE cell apoptosis in C57BL/6 J mice. Rapid OAβ(1‐42)‐induced disorganization of cytoskeletal actin filaments was accompanied by decreased RPE expression of the tight junction proteins occludin and zonula occludens‐1 and of the visual cycle proteins cellular retinaldehyde‐binding protein and RPE65. The number of photoreceptors decreased by half within a few days. Our study pinpoints the role of Aβ in RPE alterations and dysfunctions leading to retinal degeneration and suggests that targeting Aβ may help develop selective methods for treating diseases involving retinal degeneration, such as AMD.