Multiple functions of the endoplasmic reticulum (ER) essentially depend on ATP within this organelle. However, little is known about ER ATP dynamics and the regulation of ER ATP import. Here we describe real-time recordings of ER ATP fluxes in single cells using an ER-targeted, genetically encoded ATP sensor. In vitro experiments prove that the ATP sensor is both Ca(2+) and redox insensitive, which makes it possible to monitor Ca(2+)-coupled ER ATP dynamics specifically. The approach uncovers a cell type-specific regulation of ER ATP homeostasis in different cell types. Moreover, we show that intracellular Ca(2+) release is coupled to an increase of ATP within the ER. The Ca(2+)-coupled ER ATP increase is independent of the mode of Ca(2+) mobilization and controlled by the rate of ATP biosynthesis. Furthermore, the energy stress sensor, AMP-activated protein kinase, is essential for the ATP increase that occurs in response to Ca(2+) depletion of the organelle. Our data highlight a novel Ca(2+)-controlled process that supplies the ER with additional energy upon cell stimulation.