Contactin-associated protein-like 2 (Caspr2) is a neurexin-like protein that has been associated with numerous neurological conditions. However, the mechanisms underlying Caspr2 function in the central nervous system remain incompletely understood. Here, we report on a functional role for Caspr2 in the developing cerebellum. Loss of Caspr2 impairs Purkinje cell dendritic development, alters cell signaling and results in motor coordination deficits. Caspr2 is highly enriched at synaptic specializations in the cerebellum. Using a proteomic approach, we identify type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) as a specific synaptic interaction partner of the Caspr2 extracellular domain (ECD) in the molecular layer (ML) of the developing cerebellum. The interaction of Caspr2 ECD with IP3R1 inhibits IP3R1-mediated changes in cellular morphology. Together, our work defines a mechanism by which Caspr2 controls the development and function of the cerebellum, and advances our understanding of how Caspr2 dysfunction might lead to specific brain disorders.