Astrocytes contribute on both development and function of synapses, but it remains unclear whether direct astrocytic contacts regulate development of individual synapses. Two-photon time-lapse imaging of astrocytic and dendritic protrusive activity revealed the correlation of astrocytic contacts with both lifetime and morphological maturation of dendritic protrusions. Astrocytic motility was essential in maturation of spines, because its suppression by manipulating Rac1-dependent signaling in astrocytes resulted in induction of longer, filopodia-like dendritic protrusions. Manipulation of ephrin/Eph-dependent neuron–astrocyte signaling suggested involvement of this signaling pathway in astrocyte-dependent stabilization of newly generated dendritic protrusions. Our data support a model in which astrocytic protrusive activity in development acts as a key local regulator for stabilization of individual dendritic protrusions and subsequent maturation into spines.