Background: Fragile X syndrome (FXS) is a genetic cause of intellectual disability and autism. Fmr1 knockout (Fmr1KO) mice, an animal model of FXS, exhibit spatial memory impairment and synapse malfunctioning in the hippocampus, with abnormal enhancement of long-term depression mediated by metabotropic glutamate receptors (mGluR-LTD). The neurotransmitter serotonin (5-HT) modulates hippocampal-dependent cognitive functions through 5-HT1A and 5-HT7 receptors, respectively impairing and improving learning; the underlying mechanisms are unknown. Methods: we used electrophysiology to test the effects of 5-HT on mGluR-LTD in wild-type and Fmr1KO mice, and immunocytochemistry and biotinylation assay to study related changes of GluR2 AMPA receptor subunit surface expression. Results: application of 5-HT or 8-OH-DPAT (a mixed 5-HT1A/5-HT7 agonist) reversed mGluR-LTD induced by DHPG, a group-I mGluR agonist, on CA1 pyramidal neurons in hippocampal slices. Reversal of mGluR-LTD by 8-OH-DPAT persisted in the presence of the 5-HT1A receptor antagonist WAY- 100635, was abolished by SB-269970 (5-HT7 receptor antagonist) and was mimicked by LP-211, a novel selective 5-HT7 receptor agonist. Consistently, 8-OH-DPAT decreased DHPG-mediated reduction of GluR2 surface expression in hippocampal slices and in cultured hippocampal neurons, an effect mimicked by LP-211 and blocked by SB-269970. In Fmr1KO mice, mGluR-LTD was abnormally enhanced; similarly to wild-type, 8-OH-DPAT reversed mGluR-LTD and decreased DHPG-induced reduction of surface AMPA receptors, an effect antagonized by SB-269970. Conclusions: 5-HT7 receptor activation reverses mGluR-induced AMPA receptor internalization and LTD both in wild-type and in Fmr1KO mice, correcting excessive mGluR-LTD. Therefore, selective activation of 5-HT7 receptors may represent a novel strategy in the therapy of FXS.