We disclose the development of a ratiometric fluorescent probe based on a benzophosphole P-oxide and its application for the detection of intracellular Na⁺ ions. Excitation by visible light induced red emission from this probe in water, which was subjected to a hypsochromic shift upon complexation with Na⁺. Based on this change, a ratiometric analysis enabled us to visualise changes in the Na⁺ concentration in living mammalian cells.