We developed a new fluorescent pH indicator from a common Hoechst 33258 fluorescent dye. Unlike the parent compound, the improved Hoechst probe, wherein two methyl substituents were introduced into the terminal phenol ring, is fluorescent in an aqueous solution at neutral pH, and showed a unidirectional wavelength shift in the fluorescence emission spectra on lowering the pH. Thus, the probe can be used as a blue/green color-changing fluorescent pH indicator in an acidic range.