Activity-induced changes in adhesion molecules may coordinate presynaptic and postsynaptic plasticity. Here, we demonstrate that β-catenin, which mediates interactions between cadherins and the actin cytoskeleton, moves from dendritic shafts into spines upon depolarization, increasing its association with cadherins. β-catenin's redistribution was mimicked or prevented by a tyrosine kinase or phosphatase inhibitor, respectively. Point mutations of β-catenin's tyrosine 654 altered the shaft/spine distribution: Y654F-β-catenin-GFP (phosphorylation-prevented) was concentrated in spines, whereas Y654E-β-catenin-GFP (phosphorylation-mimic) accumulated in dendritic shafts. In Y654F-expressing neurons, the PSD-95 or associated synapsin-I clusters were larger than those observed in either wild-type-β-catenin or also Y654E-expressing neurons. Y654F-expressing neurons exhibited a higher minifrequency. Thus, neural activity induces β-catenin's redistribution into spines, where it interacts with cadherin to influence synaptic size and strength.