A new pH-sensitive polymeric sensor with dispersion stability and biocompatibility is synthesized, and its pH sensitivity is examined on the basis of the fluorescence resonance energy transfer (FRET) efficiency. The polymeric pH sensor has a FRET donor and a FRET acceptor attached to both ends of a pH-sensitive polymeric linker and is also PEGylated to enhance the dispersion stability in aqueous media and biocompatibility. The pH sensor emits the blue color corresponding to the emission of the FRET donor at pHs higher than 7.6, but the pH sensor emits the green color corresponding to the emission of the FRET acceptor at pHs lower than 6.8 when both samples are irradiated at 330 nm, indicating that lowering the pH from 7.6 to 6.8 induces the FRET due to the conformational change of polymeric linker from coil to globule.