Homeostatic mechanisms are required to control formation and maintenance of synaptic connections to maintain the general level of neural impulse activity within normal limits. How genes controlling these processes are coordinately regulated during homeostatic synaptic plasticity is unknown. Micro RNAs (miRNAs) exert regulatory control over mRNA stability and translation and may contribute to local activity-dependent post-transcriptional control of synapse-associated mRNAs. Using a bioinformatics screen to identify sequence motifs enriched in the 3′UTR of mRNAs that are rapidly destabilized after increasing impulse activity in hippocample neurons in culture, we identified a developmentally and activity-regulated miRNA (miR-485); we found that miR-485 controls dendritic spine number and synapse formation in an activity-dependent, homeostatic manner. Many plasticity-associated genes contain predicted miR-485 binding sites, including the presynaptic protein SV2A. We found that miR-485 reduces SV2A abundance and negatively regulates dendritic spine density, postsynaptic density protein (PSD-95) clustering, and surface expression of GluR2. Over-expression of miR-485 reduces spontaneous synaptic responses and transmitter release, as measured by miniature excitatory postsynaptic current analysis and FM 1-43 staining. The findings show that miRNAs participate in homeostatic synaptic plasticity with possible implications for neurological disorders such as Huntington's and Alzheimer's disease, in which miR-485 has been found to be dysregulated.