Glutathione peroxidases (GPXs) are enzymes that are present in almost all organisms with the primary function of limiting peroxide accumulation. In mammals, two of the eight members (GPX7 and GPX8) reside in the endoplasmic reticulum (ER). A peculiar feature of GPX8 is the concomitant presence of a conserved N-terminal transmembrane domain (TMD) and a C-terminal KDEL-like motif for ER localization. Aims: Investigating whether and how GPX8 impacts Ca 2+ homeostasis and signaling. Results: We show that GPX8 is enriched in mitochondria-associated membranes and regulates Ca 2+ storage and fluxes. Its levels correlate with [Ca 2+ ] ER , and cytosolic and mitochondrial Ca 2+ fluxes. GPX7, which lacks a TMD, does not share these properties. Deleting or replacing the GPX8 TMD with an unrelated N-terminal membrane integration sequence abolishes all effects on Ca 2+ fluxes, whereas appending the GPX8 TMD to GPX7 transfers the Ca 2+ -regulating properties. Innovation and Conclusion: The notion that the TMD of GPX8, in addition to its enzymatic activity, is essential for regulating Ca 2+ dynamics reveals a novel level of integration between redox-related proteins and Ca 2+ signaling/homeostasis. © Copyright 2017, Mary Ann Liebert, Inc.