Leukocyte common antigen–related (LAR) family receptor protein tyrosine phosphatases (LAR-RPTP) bind to liprin-α (SYD2) and are implicated in axon guidance. We report that LAR-RPTP is concentrated in mature synapses in cultured rat hippocampal neurons, and is important for the development and maintenance of excitatory synapses in hippocampal neurons. RNA interference (RNAi) knockdown of LAR or dominant-negative disruption of LAR function results in loss of excitatory synapses and dendritic spines, reduction of surface AMPA receptors, impairment of dendritic targeting of the cadherin–β-catenin complex, and reduction in the amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs). Cadherin, β-catenin and GluR2/3 are tyrosine phosphoproteins that coimmunoprecipitate with liprin-α and GRIP from rat brain extracts. We propose that the cadherin-β-catenin complex is cotransported with AMPA receptors to synapses and dendritic spines by a mechanism that involves binding of liprin-α to LAR-RPTP and tyrosine dephosphorylation by LAR-RPTP.