The extracellular matrix (ECM) is a rich collection of macromolecules that influences numerous cellular functions; however, its roles at neuronal synapses are not fully understood. Using dendritic spines of Caenorhabditis elegans GABAergic neurons as a model, we found that the ECM component Collagen XVIII/ CLE-1 is localized in close proximity to dendritic spines and is important for their normal development and maintenance. Specific expression of cle-1 in GABAergic neurons partially rescued the reduction in spine number in cle-1 ( cg120 ) mutants . Together, our findings suggest that Collagen XVIII/ CLE-1 regulates dendritic spines, in part through local CLE-1 deposition from GABAergic neurons.