Histidinemia is a congenital metabolic disorder where the histidine (His) metabolism is blocked, resulting in increased concentrations of His in blood and urine. The disease causes an abnormal development of the patient's nervous system, which leads to many serious illnesses. Therefore, it is very important to diagnose early. In this study, we developed a novel fluorescent nanosensor NaGdF₄:Yb³⁺, Er³⁺@SiO₂–spiropyran (UCNP@SiO₂–SP). The nanosensor displayed a “turn-off” fluorescence response towards His. When His was mixed with UCNP@SiO₂–SP, His could specifically bind to SP, which could cause the isomerization of SP. The structure of SP was changed from spiroform into merocyanine form. The luminescence of the sensor was overlapped with the absorption of the merocyanine form. As a result, His will lead to fluorescence quenching of the sensor based on inner filter effects (IFE), which can be used to detect His. Importantly, as the first report of a UCNP@SiO₂–SP nanosensor for detecting His, this method exhibits good selectivity and anti-interference capability. The detection limit is 4.4 μM. In addition, the amount of His in urine was also measured, suggesting the applicability of this sensor for histidinemia diagnosis.