In addition to the fundamental role of the extracellular glycoprotein Reelin in neuronal development and adult synaptic plasticity, alterations in Reelin-mediated signaling have been suggested to contribute to neuronal dysfunction associated with Alzheimer's disease (AD). In vitro data revealed a biochemical link between Reelin-mediated signaling, Tau phosphorylation, and amyloid precursor protein (APP) processing. To directly address the role of Reelin in amyloid-␤ plaque and Tau pathology in vivo, we crossed heterozygous Reelin knock-out mice (reeler) with transgenic AD mice to investigate the temporal and spatial AD-like neuropathology. We demonstrate that a reduction in Reelin expression results in enhanced amyloidogenic APP processing, as indicated by the precocious production of amyloid-␤ peptides, the significant increase in number and size of amyloid-␤ plaques, as well as age-related aggravation of plaque pathology in double mutant compared with single AD mutant mice of both sexes. Numerous amyloid-␤ plaques accumulate in the hippocampal formation and neocortex of double mutants, precisely in layers with strongest Reelin expression and highest accumulation of Reelin plaques in aged wild-type mice. Moreover, concentric accumulations of phosphorylated Tau-positive neurons around amyloid-␤ plaques were evident in 15-month-old double versus single mutant mice. Silver stainings indicated the presence of neurofibrillary tangles, selectively associated with amyloid-␤ plaques and dystrophic neurites in the entorhinal cortex and hippocampus. Our findings suggest that age-related Reelin aggregation and concomitant reduction in Reelin-mediated signaling play a proximal role in synaptic dysfunction associated with amyloid-␤ deposition, sufficient to enhance Tau phosphorylation and tangle formation in the hippocampal formation in aged Reelin-deficient transgenic AD mice.