A new ratiometric fluorescent pH nanosensor is presented. It is based on ultrabright nanoparticles containing two spatially separated fluorophores: BODIPY covalently linked to the polystyrene core and fluorescein grafted to the nanoparticle shell. The nanoparticles comprise a large number (≥2500) of both fluorescent moieties. Their spectroscopic characteristics were studied at different pH and ionic strength. They could successfully be used to determine the solution pH between 5.5 and 7.5 by measuring the fluorescence intensity ratio of the sensor molecule (fluorescein) relative to the reference dye (BODIPY).