In the last decades, poly(methyl methacrylate) (PMMA) has been widely studied due to its outstanding mechanical, chemical, and physical properties. In this article, the PMMA composites doped with an excited state intramolecular proton transfer (ESIPT) compound 3-hydroxy-2-naphthoic acid (3HNA) are reported. The 3HNA-doped PMMA composites were synthesized by in situ polymerization and characterized by Fourier transform infrared spectra, ultraviolet absorption spectra, fluorescence spectra, and thermogravimetric analysis. The effect of 3HNA and various solvents on the fluorescent properties of the PMMA composites was systematically investigated. The results indicated that the emission spectra of the 3HNA-doped PMMA composites exhibited 3HNA dual emission. A violet emission was observed in 3HNA-doped PMMA composites containing nonpolar solvents or weak polar solvents, whereas the composites containing strong polar solvents exhibited a large Stokes-shifted green fluorescence, which was attributed to the ESIPT of 3HNA.