TRPM2 is a Ca²⁺-permeable cation channel that is specifically activated by adenosine diphosphoribose (ADPR). Channel activation in the plasma membrane leads to Ca²⁺ influx and has been linked to apoptotic mechanisms. The primary agonist, ADPR, is produced both extra- and intracellularly and causes increases in intracellular calcium concentration ([Ca²⁺]_i), but the mechanisms involved are not understood. Using short interfering RNA and a knockout mouse, we report that TRPM2, in addition to its role as a plasma membrane channel, also functions as a Ca²⁺-release channel activated by intracellular ADPR in a lysosomal compartment. We show that both functions of TRPM2 are critically linked to hydrogen peroxide–induced β cell death. Additionally, extracellular ADPR production by the ectoenzyme CD38 from its substrates NAD⁺ (nicotinamide adenine dinucleotide) or cADPR causes IP₃-dependent Ca²⁺ release via P2Y and adenosine receptors. Thus, ADPR and TRPM2 represent multimodal signaling elements regulating Ca²⁺ mobilization in β cells through membrane depolarization, Ca²⁺ influx, and release of Ca²⁺ from intracellular stores.