Antiapoptotic Bcl-2 family members interact with inositol trisphosphate receptor (InsP₃R) Ca2+ release channels in the endoplasmic reticulum to modulate Ca2+ signals that affect cell viability. However, the molecular details and consequences of their interactions are unclear. Here, we found that Bcl-xL activates single InsP₃R channels with a biphasic concentration dependence. The Bcl-xL Bcl-2 homology 3 (BH3) domain-binding pocket mediates both high-affinity channel activation and low-affinity inhibition. Bcl-xL activates channel gating by binding to two BH3 domain-like helices in the channel carboxyl terminus, whereas inhibition requires binding to one of them and to a previously identified Bcl-2 interaction site in the channel-coupling domain. Disruption of these interactions diminishes cell viability and sensitizes cells to apoptotic stimuli. Our results identify BH3-like domains in an ion channel and they provide a unifying model of the effects of antiapoptotic Bcl-2 proteins on the InsP₃R that play critical roles in Ca2+ signaling and cell viability.