IRBIT has previously been shown to interact with the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) in an IP₃-sensitive way. So far it remained to be elucidated whether this interaction was direct or indirect, and whether it was functionally relevant. We now show that IRBIT can directly interact with the IP₃R, and that both the suppressor domain and the IP₃-binding core of the IP₃R are essential for a strong interaction. Moreover, we identified a PEST motif and a PDZ-ligand on IRBIT which were critical for the interaction with the IP₃R. Furthermore, we identified Asp-73 as a critical residue for this interaction. Finally, we demonstrated that this interaction functionally affects the IP₃R: IRBIT inhibits both IP₃ binding and IP₃-induced Ca²⁺ release.