<p>Tight regulation of mitochondrial Ca²⁺ is essential for neuronal bioenergetics and cellular metabolism. Ca²⁺ transfer from ER-localized ryanodine receptors (RyR) and inositol triphosphate receptors (IP₃R) to the mitochondria maintains a steady Ca²⁺ source that fuels oxidative phosphorylation and ATP production. In Alzheimer’s disease (AD), RyR-evoked Ca²⁺ release is markedly increased, contributing to synaptic deficits, protein mishandling, and memory impairment. Here, we demonstrate dysregulated RyR-Ca²⁺ release in neurons from familial and sporadic AD patients, and this directly compromises mitochondrial activity and contributes to AD cellular pathology. We measured an array of mitochondrial functions using fluorescent biosensors and optical imaging in RyR2-expressing HEK cells and neurons derived from AD and nonAD individuals. In neurons from AD patients, resting mitochondrial Ca²⁺ levels were elevated alongside increased free radical production and higher caspase-3 activity relative to nonAD neurons. RyR-evoked Ca²⁺ release further potentiated pathogenic mitochondrial responses in AD neurons, with increased Ca²⁺ uptake and exaggerated mitochondrial membrane depolarization. Additionally, clearance of damaged mitochondria was impaired in AD neurons, demonstrating consequences from dysfunctional lysosomes. Notably, impairments to mitochondria in AD neurons were largely prevented with the RyR negative allosteric modulator, Ryanodex. These findings highlight how excess RyR-Ca²⁺ release broadly contributes to early cellular pathology in AD which includes a cascade of ER, lysosomal, and mitochondrial deficits culminating in neuronal decline and degeneration. Additionally, pharmacological suppression of RyR-Ca²⁺ release preserves mitochondrial, ER and lysosomal function, thus providing a novel and effective therapeutic strategy.</p>