Activity-dependent protein synthesis at synapses is dysregulated in the Fragile X syndrome (FXS). This process contributes to dendritic spine dysmorphogenesis and synaptic dysfunction in FXS. Matrix Metalloproteinase 9 (MMP-9) is an enzyme involved in activity-dependent reorganization of dendritic spine architecture and was shown to regulate spine morphology in a mouse model of FXS, the Fmr1 knock-out mice. Here we show that MMP-9 mRNA is part of the FMRP complex and colocalizes in dendrites. In the absence of FMRP MMP-9 mRNA translation is increased at synapses, suggesting that this mechanism contributes to the increased metalloproteinase level at synapses of Fmr1 knock-out mice. We propose that such a local effect can contribute to the aberrant dendritic spine morphology observed in the Fmr1 knock-out mice and in patients with FXS.