A novel pH-sensitive near-IR (NIR) fluorescent dye was synthesised and utilised in the preparation of fluorescent polystyrene microspheres. Their photochemical properties, as well as the pH-dependent optical properties were investigated and showed high sensitivity to changes of pH in the range of 3.3–8.1. The fluorescence became stronger with the increase in pH. Importantly, the microspheres were able to work in aqueous media, which is highly valuable for use in biological applications.