At vertebrate neuromuscular junctions (NMJs), Agrin plays pivotal roles in synapse development, but molecules that activate synapse formation at central synapses are largely unknown. Members of the Wnt family are well established as morphogens, yet recently they have also been implicated in synapse maturation. Here we demonstrate that the Drosophila Wnt, Wingless (Wg), is essential for synapse development. We show that Wg and its receptor are expressed at glutamatergic NMJs, and that Wg is secreted by synaptic boutons. Loss of Wg leads to dramatic reductions in target-dependent synapse formation, and new boutons either fail to develop active zones and postsynaptic specializations or these are strikingly aberrant. We suggest that Wg signals the coordinated development of pre- and postsynaptic compartments.