By transiently or stably overexpressing the mitochondrial fission factor dynamin-related protein-1 (Drp-1), we evaluated the role of mitochondrial division in organelle Ca²⁺ homeostasis and apoptotic signaling. Quantitative 3D digital microscopy revealed a split mitochondrial network in Drp-1-overexpressing cells without changes in cell viability. High-speed mitochondrial [Ca²⁺] ([Ca²⁺]_m) imaging revealed propagating intramitochondrial Ca²⁺ waves in intact cells, which were blocked in the Drp-1-fragmented network, leaving a fraction of individual mitochondria without substantial [Ca²⁺]_m elevation. Consequently, in Drp-1-expressing cells the apoptotic efficacy of ceramide, which causes a Ca²⁺-dependent perturbation of mitochondrial structure and function, was drastically reduced. Conversely, the sensitivity to staurosporine-induced apoptosis, previously shown to be directly triggered by Drp-1-dependent recruitment of proapoptotic proteins to mitochondria, was enhanced. These results demonstrate that the regulated process of mitochondrial fusion and fission controls the spatiotemporal properties of mitochondrial Ca²⁺ responses and, thus, physiological and pathological consequences of cellular Ca²⁺ signals.