Understanding the physical and chemical properties of individual ions requires methods that can probe their behaviour and dynamics in liquid environments at the single-ion level, yet such measurement remains challenging. Here we use the optical emission of defects in hexagonal boron nitride (h-BN) as single-ion sensors. Interactions between optically active h-BN defects and ions produce distinct spectral shifts, enabling the spectroscopic detection of individual ions. Using Li+ in organic electrolytes as a model, we observed spectral shifts exceeding 10 nm upon ion addition. Applying an external electric field increased these shifts to more than 40 nm, enabling real-time monitoring of local ion perturbations. Individual defects further distinguished Na+, Mg2+ and Al3+ through their characteristic spectral shifts. This platform enables direct sensing of individual ions in liquids with millisecond temporal resolution at concentrations down to the 10 micromolar range, providing a route to probing interfacial processes relevant to batteries and environmental systems.