At Caenorhabditis elegans neuromuscular junctions (NMJs), synaptic clustering of the levamisole-sensitive acetylcholine receptors (L-AChRs) relies on an extracellular scaffold assembled in the synaptic cleft. It involves the secreted protein LEV-9 and the ectodomain of the transmembrane protein LEV-10, which are both expressed by muscle cells. L-AChRs, LEV-9 and LEV-10 are part of a physical complex, which localizes at NMJs, yet none of its components localizes independently at synapses. In a screen for mutants partially resistant to the cholinergic agonist levamisole, we identified oig-4, which encodes a small protein containing a single immunoglobulin domain. The OIG-4 protein is secreted by muscle cells and physically interacts with the L-AChR/LEV-9/LEV-10 complex. Removal of OIG-4 destabilizes the complex and causes a loss of L-AChR clusters at the synapse. Interestingly, OIG-4 partially localizes at NMJs independently of LEV-9 and LEV-10, thus providing a potential link between the L-AChR-associated scaffold and local synaptic cues. These results add a novel paradigm for the immunoglobulin super-family as OIG-4 is a secreted protein required for clustering ionotropic receptors independently of synapse formation.