Organelle-selective oxygen probe BTP-Mito was designed and synthesized to selectively target mitochondria. BTP-Mito, which is an iridium complex bearing a triphenylphosphonium cation, exhibited selective mitochondria localization in HeLa cells. The phosphorescence of BTP-Mito was significantly quenched by molecular oxygen in living cells, demonstrating that BTP-Mito can be used as mitochondria-specific oxygen sensor.