A novel fluorescent HCO3− probe based on urea–thiadiazole–benzene (UTB) fluorescent organic nanoparticles (FONs) were designed and synthesized. UTB FONs exhibit excellent water solubility, high selectivity for sensing upon binding HCO3− in aqueous media. Under the optimum conditions, the relative fluorescent intensity is decreased linearly with increasing bicarbonate concentration in the range of 0–1000 μM with a detection limit of 5.57 × 10−7 M. It is found that the decreased luminescence intensity of the UTB FONs in a concentration of HCO3− ions dependence is best described by a Stern–Volmer equation. The possible mechanism is discussed.