The inositol trisphosphate (InsP₃) receptor (InsP₃R) is a ubiquitously expressed intracellular Ca²⁺ channel that mediates complex cytoplasmic Ca²⁺ signals, regulating diverse cellular processes, including synaptic plasticity. Activation of the InsP₃R channel is normally thought to require binding of InsP₃ derived from receptor-mediated activation of phosphatidylinositol lipid hydrolysis. Here we identify a family of neuronal Ca²⁺-binding proteins as high-affinity protein agonists of the InsP₃R, which bind to the channel and activate gating in the absence of InsP₃. CaBP/caldendrin, a subfamily of the EF-hand-containing neuronal calcium sensor family of calmodulin-related proteins, bind specifically to the InsP₃-binding region of all three InsP₃R channel isoforms with high affinity (K_a ≈ 25 nM) in a Ca²⁺-dependent manner (K_a ≈ 1 μM). Binding activates single-channel gating as efficaciously as InsP₃, dependent on functional EF-hands in CaBP. In contrast, calmodulin neither bound with high affinity nor activated channel gating. CaBP1 and the type 1 InsP₃R associate in rat whole brain and cerebellum lysates, and colocalize extensively in subcellular regions in cerebellar Purkinje neurons. Thus, InsP₃R-mediated Ca²⁺ signaling in cells is possible even in the absence of InsP₃ generation, a process that may be particularly important in responding to and shaping changes in intracellular Ca²⁺ concentration by InsP₃-independent pathways and for localizing InsP₃-mediated Ca²⁺ signals to individual synapses.