Alterations of adult neurogenesis have been reported in several Alzheimer's disease (AD) animal models and human brains, while defects in this process at presymptomatic/early stages of AD have not been explored yet. To address this, we investigated potential neurogenesis defects in Tg2576 transgenic mice at 1.5 months of age, a prodromal asymptomatic age in terms of Aβ accumulation and neurodegeneration. We observe that Tg2576 resident and SVZ-derived adult neural stem cells (aNSCs) proliferate significantly less. Further, they fail to terminally differentiate into mature neurons due to pathological, tau-mediated, and microtubule hyperstabilization. Olfactory bulb neurogenesis is also strongly reduced, confirming the neurogenic defect in vivo. We find that this phenotype depends on the formation and accumulation of intracellular A-beta oligomers (AβOs) in aNSCs. Indeed, impaired neurogenesis of Tg2576 progenitors is remarkably rescued both in vitro and in vivo by the expression of a conformation-specific anti-AβOs intrabody (scFvA13-KDEL), which selectively interferes with the intracellular generation of AβOs in the endoplasmic reticulum (ER). Altogether, our results demonstrate that SVZ neurogenesis is impaired already at a presymptomatic stage of AD and is caused by endogenously generated intracellular AβOs in the ER of aNSCs. From a translational point of view, impaired SVZ neurogenesis may represent a novel biomarker for AD early diagnosis, in association to other biomarkers. Further, this study validates intracellular Aβ oligomers as a promising therapeutic target and prospects anti-AβOs scFvA13-KDEL intrabody as an effective tool for AD treatment.