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Summary: The transport and translation of dendritic mRNAs by RNA-binding proteins (RBPs) allows for spatially restricted gene expression in neuronal processes. Although local translation in neuronal dendrites is now well documented, there is little evidence for corresponding effects on local synaptic function. Here, we report that the RBP Sam68 promotes the localization and translation of Arc mRNA preferentially in distal dendrites of rodent hippocampal CA1 pyramidal neurons. Consistent with Arc function in translation-dependent synaptic plasticity, we find that Sam68 knockout (KO) mice display impaired metabotropic glutamate-receptor-dependent long-term depression (mGluR-LTD) and impaired structural plasticity exclusively at distal Schaffer-collateral synapses. Moreover, by using quantitative proteomics, we find that the Sam68 interactome contains numerous regulators of mRNA translation and synaptic function. This work identifies an important player in Arc expression, provides a general framework for Sam68 regulation of protein synthesis, and uncovers a mechanism that enables the precise spatiotemporal expression of long-term plasticity throughout neurons. : Although local translation in neuronal dendrites is well documented, there is little evidence for corresponding effects on local synaptic function. Klein et al. demonstrate that Sam68 is required for Arc protein synthesis at distal dendritic regions and is required for synaptic plasticity exclusively at distal dendrites of hippocampal pyramidal neurons. Keywords: RNA-binding proteins, mRNA localization, Arc, Arg3.1, local translation, interactome, isobaric tags, proteomic, RNAscope