Dendritic spine morphology is strongly associated with neurodevelopmental disorders. Synaptic plasticity alters spine volume, a phenomenon known as structural plasticity, which influences information processing within neuronal circuits. Structural changes at dendritic spines are linked to autism spectrum disorders, particularly those involving gene mutations that result in synaptopathy. Loss of a single copy of the Shank3 gene leads to Phelan-McDermid syndrome, a synaptopathy, as Shank3 encodes SHANK3, a scaffold protein in the postsynaptic density of glutamatergic neurons. In this study, the structural plasticity of dendritic spines was evaluated in male and female Shank3+/- and wild-type mice in response to synaptic plasticity. Two-photon imaging and glutamate uncaging were employed in organotypic hippocampal cultures. Cognitive function in adult Shank3+/- mice was also assessed using a novel object recognition test. The results indicate that Shank3+/- mice exhibit altered structural plasticity in response to long-term depression and display a heterosynaptic response in neighboring spines. Increased GluN2B expression and N-methyl-d-aspartate currents underlie these effects and may influence object recognition memory in Shank3+/- mice. These findings suggest that Shank3 haploinsufficiency induces synaptic alterations during postnatal development that impact memory in adulthood.