Contactin-associated protein-like 2 (Caspr2) is a neurexin-like protein that has been associated with numerous neurological conditions. However, the specific functional roles that Caspr2 plays in the central nervous system and their underlying mechanisms remain incompletely understood. Here, we report on a functional role for Caspr2 in the developing cerebellum. Using a combination of confocal microscopy, biochemical analyses and behavioral testing, we show that loss of Caspr2 in the Cntnap2-/- knockout mouse results in impaired Purkinje cell dendritic development, altered intracellular signaling and motor coordination deficits. We also find that Caspr2 is highly enriched at synaptic specializations in the cerebellum. Using a proteomics 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.