A crucial issue in neurobiology is to understand the main mechanisms restricting neural plasticity to brief windows of early postnatal life. The visual system is one of the paradigmatic models for studying experience-dependent plasticity. The closure of one eye (monocular deprivation, MD) causes a marked ocular dominance (OD) shift of neurons in the primary visual cortex only during the critical period. Here, we report that environmental enrichment (EE), a condition of increased sensory-motor stimulation, reactivates OD plasticity in the adult visual cortex, as assessed with both visual evoked potentials and single-unit recordings. This effect is accompanied by a marked increase in cerebral serotonin (5-HT) levels. Blocking 5-HT enhancement in the visual cortex of EE rats completely prevents the OD shift induced by MD. We also found that EE leads to a reduced intracortical GABAergic inhibition and an increased BDNF expression and that the modulation of these molecular factors is neutralized by cortical infusion of the 5-HT synthesis inhibitor pCPA. Our results show that EE rejuvenates the adult visual cortex and that 5-HT is a crucial factor in this process, triggering a cascade of molecular events that allow the reinstatement of neural plasticity. The non-invasive nature of EE makes this paradigm particularly eligible for clinical application.